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Basis of Presentation

A. Basis of Presentation

The consolidated financial statements have been prepared in conformity with accounting policies generally accepted in the United States (“U.S.”) and include the accounts of Cabot Corporation (“Cabot” or the “Company”) and its wholly owned subsidiaries and majority-owned and controlled U.S. and non-U.S. subsidiaries. Additionally, Cabot considers consolidation of entities over which control is achieved through means other than voting rights. Intercompany transactions have been eliminated in consolidation.

The unaudited consolidated financial statements have been prepared in accordance with the requirements of Form 10-Q and consequently do not include all disclosures required by Form 10-K. Additional information may be obtained by referring to Cabot’s Annual Report on Form 10-K for its fiscal year ended September 30, 2018 (“2018 10-K”).

The financial information submitted herewith is unaudited and reflects all adjustments which are, in the opinion of management, necessary to provide a fair statement of the results for the interim periods ended June 30, 2019 and 2018. All such adjustments are of a normal recurring nature. The results for interim periods are not necessarily indicative of the results to be expected for the fiscal year.

Effective October 1, 2018, the Company adopted the new revenue recognition standard that amended the pre-existing accounting standard for revenue recognition. The Company used a modified retrospective approach, which is discussed in further detail in Note B.

Effective October 1, 2018, the Company adopted the new pension standard that amended the requirements on the presentation of net periodic pension and postretirement benefit costs. The Company used a retrospective transition method to reclassify net periodic benefit cost, other than the service component, from Cost of sales, Selling and administrative expenses and Research and technical expenses to Other income (expense) in the Consolidated Statements of Operations for the prior periods presented, which is discussed in further detail in Note B.

Effective October 1, 2018, the Company realigned its business reporting structure under the Performance Chemicals segment and now combines its specialty carbons, fumed metal oxides and aerogel product lines into the Performance Additives business, and its specialty compounds and inkjet product lines into the Formulated Solutions business. Prior period Performance Chemicals segment revenue results have been recast to reflect the realignment.

In January 2019, the Company entered into an agreement to sell its Specialty Fluids business to Sinomine (Hong Kong) Rare Metals Resources Co. Limited, a wholly owned subsidiary of Sinomine Resource Group Co., Ltd. (“Sinomine”). The sale of the Specialty Fluids business closed in June 2019 and does not meet the criteria to be reported as a discontinued operation. Therefore, prior periods’ consolidated financial statements and disclosures have not been recast. Refer to Note D for further details regarding the sale of the Specialty Fluids business.

Significant Accounting Policies

9 Months Ended

Jun. 30, 2019

Significant Accounting Policies

B. Significant Accounting Policies

Revenue Recognition

Cabot recognizes revenue when its customers obtain control of promised goods or services. The revenue recognized is the amount of consideration which the Company expects to receive in exchange for those goods or services. Cabot derives the majority of its revenues from contracts for the sale of products from its Reinforcement Materials, Performance Chemicals and Purification Solutions segments. The Company’s contracts with customers are generally for products only and do not include other performance obligations. Generally, Cabot considers purchase orders, which in some cases are governed by master supply agreements, to be contracts with customers. The transaction price as specified on the purchase order or sales contract is considered the standalone selling price for each distinct product. To determine the transaction price at the time when revenue is recognized, the Company evaluates whether the price is subject to adjustments, such as for returns, discounts or volume rebates, which are stated in the customer contract, to determine the net consideration to which the Company expects to be entitled. Revenue from product sales is recognized based on a point in time model when control of the product is transferred to the customer, which typically occurs upon shipment or delivery of the product to the customer and title, risk and rewards of ownership have passed to the customer. Payment terms typically range from zero to ninety days.

Shipping and handling activities that occur after the transfer of control to the customer are billed to customers and are recorded as sales revenue, as the Company considers these to be fulfillment costs. Shipping and handling costs are expensed in the

period incurred and included in Cost of sales within the Consolidated Statement of Operations. Taxes collected on sales to customers are excluded from the transaction price.

The Company generally provides a warranty that its products will substantially conform to the identified specifications. The Company’s liability typically is limited to either a credit equal to the purchase price or replacement of the non-conforming product. Returns under warranty have historically been immaterial.

Revenue in the Specialty Fluids segment arose primarily from the rental of cesium formate. This revenue was recognized throughout the rental period based on the contracted rental terms. Customers were also billed and revenue was recognized, typically at the end of the rental period, for cesium formate product that was not returned. The Company also generated revenues from cesium formate sold outside of the rental process and from the sale of fine cesium chemicals. This revenue was recognized when control of the product transferred to the customer, which was typically upon delivery of the product.

The Company does not have contract assets or liabilities that are material.

As permitted by the revenue recognition standard, Revenue from Contracts with Customers, when the period of time between the transfer of control of the goods and the time the customer pays for the goods is one year or less, the Company does not consider there to be a significant financing component associated with the contract.

Intangible Assets and Goodwill Impairment

The Company records tangible and intangible assets acquired and liabilities assumed in business combinations under the acquisition method of accounting. Amounts paid for an acquisition are allocated to the assets acquired and liabilities assumed based on their fair values at the date of acquisition. The Company uses assumptions and estimates in determining the fair value of assets acquired and liabilities assumed in a business combination. The determination of the fair value of intangible assets requires the use of significant judgment with regard to assumptions used in the valuation model. The Company estimates the fair value of identifiable acquisition-related intangible assets principally based on projections of cash flows that will arise from these assets. The projected cash flows are discounted to determine the fair value of the assets at the dates of acquisition.

Definite-lived intangible assets, which are comprised of trademarks, customer relationships and developed technologies, are amortized over their estimated useful lives and are reviewed for impairment when indication of potential impairment exists, such as a significant reduction in cash flows associated with the assets.

Goodwill is comprised of the purchase price of business acquisitions in excess of the fair value assigned to the net tangible and identifiable intangible assets acquired. Goodwill is not amortized, but is reviewed for impairment annually as of May 31, or when events or changes in the business environment indicate that the carrying value of the reporting unit may exceed its fair value. A reporting unit, for the purpose of the impairment test, is at or below the operating segment level, and constitutes a business for which discrete financial information is available and regularly reviewed by segment management. Reinforcement Materials, and the fumed metal oxides and specialty compounds product lines within Performance Chemicals, which are considered separate reporting units, carry the Company’s goodwill balances as of June 30, 2019.

For the purpose of the goodwill impairment test, the Company first assesses qualitative factors to determine whether it is more likely than not that the fair value of a reporting unit is less than its carrying amount. If an initial qualitative assessment identifies that it is more likely than not that the carrying value of a reporting unit exceeds its estimated fair value, an additional quantitative evaluation is performed. Alternatively, the Company may elect to proceed directly to the quantitative goodwill impairment test. If based on the quantitative evaluation the fair value of the reporting unit is less than its carrying amount, a goodwill impairment loss would result. The goodwill impairment loss would be the amount by which the carrying value of the reporting unit, including goodwill, exceeds its fair value, limited to the total amount of goodwill allocated to that reporting unit. The fair value of a reporting unit is based on discounted estimated future cash flows. The fair value is also benchmarked against a market approach using the guideline public companies method. The assumptions used to estimate fair value include management’s best estimates of future growth rates, operating cash flows, capital expenditures and discount rates over an estimate of the remaining operating period at the reporting unit level. Based on the Company’s most recent annual goodwill impairment test performed as of May 31, 2019, the fair values of the Reinforcement Materials, Fumed Metal Oxides and Specialty Compounds reporting units were substantially in excess of their carrying values. Refer to Note F for details on the Purification Solutions goodwill impairment test and the resulting impairment charge recorded in the second quarter of fiscal 2018.

Long-lived Assets Impairment

The Company’s long-lived assets primarily include property, plant and equipment, intangible assets, long-term investments and assets held for rent. The carrying values of long-lived assets are reviewed for impairment whenever events or changes in business circumstances indicate that the carrying amount of an asset may not be recoverable.

To test for impairment of assets, the Company generally uses a probability-weighted estimate of the future undiscounted net cash flows of the assets over their remaining lives to determine if the value of the asset is recoverable. Long-lived assets are grouped with other assets and liabilities at the lowest level for which independent identifiable cash flows are determinable.

An asset impairment is recognized when the carrying value of the asset is not recoverable based on the analysis described above, in which case the asset is written down to its fair value. If the asset does not have a readily determinable fair value, a discounted cash flow model may be used to determine the fair value of the asset. In circumstances when an asset does not have separately identifiable cash flows, an impairment charge is recorded when the Company no longer intends to use the asset. Refer to Note F regarding the results of the recoverability test performed on the long-lived assets of the Purification Solutions segment and the resulting impairment charge recorded in the second quarter of fiscal 2018.

The Company continues to pursue strategic options for its Purification Solutions business. Depending on the actions taken, there could be a negative impact on the fair value of the Purification Solutions reporting unit, which may lead to further impairment.

Property, Plant and Equipment

Property, plant and equipment are recorded at cost. Depreciation of property, plant and equipment is calculated using the straight-line method over the estimated useful lives of the related assets. The depreciable lives for buildings, machinery and equipment, and other fixed assets are between twenty and twenty-five years, ten and twenty-five years, and three and twenty-five years, respectively. The cost and accumulated depreciation for property, plant and equipment sold, retired, or otherwise disposed of are removed from the Consolidated Balance Sheets and resulting gains or losses are included in earnings in the Consolidated Statements of Operations. Expenditures for repairs and maintenance are charged to expenses as incurred. Expenditures for major renewals and betterments, which significantly extend the useful lives of existing plant and equipment, are capitalized and depreciated.

Income Tax in Interim Periods

The Company records its tax provision or benefit on an interim basis using an estimated annual effective tax rate. This rate is applied to the current period ordinary income or loss to determine the income tax provision or benefit allocated to the interim period. Losses from jurisdictions for which no benefit can be recognized and the income tax effects of unusual or infrequent items are excluded from the estimated annual effective tax rate and are recognized in the impacted interim period.

Valuation allowances are provided against the future tax benefits that arise from the deferred tax assets in jurisdictions for which no benefit can be recognized. The estimated annual effective tax rate may be significantly impacted by nondeductible expenses and the Company’s projected earnings mix by tax jurisdiction. Adjustments to the estimated annual effective income tax rate are recognized in the period when such estimates are revised.

Inventory Valuation

Inventories are stated at the lower of cost or net realizable value. The cost of Specialty Fluids inventories that were classified as assets held for rent was determined using the average cost method. The cost of all other inventories is determined using the FIFO method.

Cabot periodically reviews inventory for both potential obsolescence and potential declines in anticipated selling prices. In this review, the Company makes assumptions about the future demand for and market value of the inventory, and based on these assumptions estimates the amount of any obsolete, unmarketable, slow moving, or overvalued inventory. Cabot writes down the value of these inventories by an amount equal to the difference between the cost of the inventory and its estimated net realizable value.

Pensions and Other Postretirement Benefits

The Company recognizes the funded status of defined benefit pension and other postretirement benefit plans as an asset or liability. This amount is defined as the difference between the fair value of plan assets and the benefit obligation. The Company is required to recognize as a component of Other comprehensive income (loss), net of tax, the actuarial gains/losses and prior service costs/credits that arise but were not previously required to be recognized as components of net periodic benefit cost. Other comprehensive income (loss) is adjusted as these amounts are later recognized in income as components of net periodic benefit cost.

Redeemable Preferred Stock

In November 2013, the Company purchased all of its joint venture partner’s common stock in the former NHUMO, S.A. de C.V. (“NHUMO”) joint venture. At the close of the transaction, NHUMO issued redeemable preferred stock to the joint venture partner with a repurchase value of $25 million and a fixed dividend rate of 6% per annum. In November 2018, the preferred stock was repurchased for $25 million and a final dividend payment of approximately $1.4 million was made. The preferred stock was accounted for as a financing obligation and was separately presented in the Consolidated Balance Sheets as a current liability as of September 30, 2018.

Accumulated Other Comprehensive Income (Loss)

Accumulated other comprehensive income (loss) (“AOCI”), which is included as a component of stockholders’ equity, includes unrealized gains or losses on available-for-sale marketable securities and derivative instruments, currency translation adjustments in foreign subsidiaries, translation adjustments on foreign equity securities and minimum pension liability adjustments.

Recently Adopted Accounting Standards

In May 2014, the Financial Accounting Standards Board (“FASB”) issued a new standard, Revenue from Contracts with Customers, that amended the existing accounting standards for revenue recognition. The standard requires entities to recognize revenue when they transfer promised goods or services to customers in an amount that reflects the consideration the entity expects to be entitled to in exchange for those goods or services. The Company adopted this standard on October 1, 2018 and applied a modified retrospective approach. The comparative information has not been restated and continues to be reported under the accounting standards in effect during those periods, as discussed in the Company’s 2018 10-K.

The adoption of this standard did not have an impact on how the Company recognizes revenue. As such, an adjustment to opening retained earnings was not required. The Company implemented the updates that were necessary to its revenue recognition policy, internal controls, processes and financial statement disclosures as part of this adoption. The updated disclosures are reflected under the heading “Revenue Recognition” within Note B and the Company’s disaggregated revenue is reflected within Note P.

In addition, as part of an assessment performed in connection with adopting this standard, the Company reviewed its classification of by-product sales, which consist of sales generated from the production of steam or electricity from the Company's energy centers primarily from its carbon black manufacturing sites and sales of hydrochloric acid generated from the production of fumed silica. Historically, the Company presented by-product sales as a reduction of Cost of sales within the Consolidated Statement of Operations. However, upon further evaluation of these sales in connection with the implementation of this standard, the Company determined that it is appropriate to present by-product sales as Net sales and other operating revenues. Effective October 1, 2018, these sales are now included within Net sales and other operating revenues in the Consolidated Statement of Operations. This change did not result in a cumulative adjustment to opening retained earnings as it is a classification change within the Consolidated Statement of Operations. If the Company had continued to classify by-product sales within Cost of sales, there would have been a decrease to Net sales and other operating revenues and Cost of sales of $18 million and $58 million for the three and nine months ended June 30, 2019, respectively.

In August 2016, the FASB issued final amendments to clarify how entities should classify certain cash receipts and cash payments on the statement of cash flows, such as distributions received from equity method investees, proceeds from the settlement of insurance claims, and proceeds from the settlement of corporate-owned life insurance policies. The Company adopted this standard on October 1, 2018. The adoption of this standard did not have a material impact to the Company’s consolidated financial statements.

In March 2017, the FASB issued a new standard that amends the requirements on the presentation of net periodic pension and postretirement benefit costs. The new standard requires the service cost component to be presented with other employee compensation costs. The other components will be reported separately outside of operations. Only the service cost component will be eligible for capitalization. Entities are required to use a retrospective transition method to adopt the requirement for separate income statement presentation of the service cost and other components, and a prospective transition method to adopt the requirement to limit the capitalization of benefit cost to the service component. The Company adopted the standard on October 1, 2018 and used a retrospective transition method to reclassify net periodic benefit cost, other than the service component, from Cost of sales, Selling and administrative expenses and Research and technical expenses to Other income (expense) in the Consolidated Statements of Operations for the prior periods presented. In accordance with the standard, the Company utilized prior period footnote disclosures as a practical expedient to apply these retrospective presentations, which is shown in the table below:

Consolidated Statements of Operations	Three Months Ended June 30								Nine Months Ended June 30
	2018								2018
	As Originally Reported			Effect of Change			As Adjusted		As Originally Reported		Effect of Change			As Adjusted
	(In millions, except per share amounts)
Cost of sales	$	654		$	3		$	657	$	1,824	$	7		$	1,831
Gross profit	$	200		$	(3	)	$	197	$	568	$	(7	)	$	561
Selling and administrative expenses	$	74		$	—		$	74	$	221	$	2		$	223
Research and technical expenses	$	17		$	—		$	17	$	48	$	—		$	48
Income (loss) from operations	$	109		$	(3	)	$	106	$	45	$	(9	)	$	36
Other income (expense)	$	(2	)	$	3		$	1	$	4	$	9		$	13

In August 2018, the FASB issued a new standard to align the requirements for capitalizing implementation costs incurred in a cloud computing arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software. The updated guidance also requires an entity to expense the capitalized implementation costs of a cloud computing arrangement that is a service contract over the term of the arrangement and includes expanded disclosure requirements for such costs. The Company adopted this standard prospectively on January 1, 2019. The adoption of this standard did not have a material impact to the Company’s consolidated financial statements.

Recent Accounting Pronouncements

In February 2016, the FASB issued a new standard for the accounting for leases. This new standard requires lessees to recognize assets and liabilities for most leases, but recognize expenses on their income statements in a manner that is similar to the current accounting treatment for leases. The standard is applicable for fiscal years beginning after December 15, 2018 and for interim periods within those years, and early adoption is permitted. The Company is currently evaluating the impact of the adoption of this standard on its consolidated financial statements. The Company expects to adopt the standard on October 1, 2019 using the modified retrospective optional transition method, in which case prior periods presented will not be restated. The Company intends to elect the package of practical expedients, which, among other things, permits the Company to not reassess the identification, classification and initial direct costs of leases commencing before the October 1, 2019 effective date and not include short-term leases on the balance sheet. The Company has established a project plan and implementation team that is analyzing the current portfolio of leases to determine the impact of adopting this new standard. The implementation team is also responsible for evaluating and designing the necessary changes to the Company’s business processes, lease policies, systems and internal controls to support recognition and disclosure under the new guidance.

In June 2016, the FASB issued a new standard on measurement of credit losses. The standard introduces a new "expected loss" impairment model that applies to most financial assets measured at amortized cost and certain other instruments, including trade and other receivables and other financial assets. Entities are required to estimate expected credit losses over the life of financial assets and record an allowance against the assets’ amortized cost basis to present them at the amount expected to be collected. The new standard is effective for fiscal years beginning after December 15, 2019 and early adoption is permitted. The Company is evaluating this standard and the timing of its adoption. The Company does not expect the adoption of this standard to materially impact the Company’s consolidated financial statements.

In February 2018, the FASB issued a new standard that allows entities to reclassify from AOCI to Retained earnings stranded tax effects resulting from changes made as a result of the Tax Cuts and Jobs Act of 2017 (the “Act”). The amendments in this new standard also require certain disclosures about stranded tax effects. The new standard is effective for all entities for fiscal years beginning after December 15, 2018, including interim periods within those years, and early adoption is permitted. The Company is evaluating this standard and the timing of its adoption. The Company does not expect the adoption of this standard to materially impact the Company’s consolidated financial statements.

Acquisitions

9 Months Ended

Jun. 30, 2019

Business Combinations [Abstract]

Acquisitions

C. Acquisitions

Tech Blend

In November 2017, the Company acquired Tech Blend, a North American producer of black masterbatches, for a purchase price of $65 million, paid in cash. The purchase price was subject to a working capital adjustment, which was immaterial. The operating results of the business are included in the Company’s Performance Chemicals segment. The acquisition extended the Company’s global footprint in black masterbatch and compounds and provides a platform to serve global customers and grow in conductive formulations.

NSCC Carbon (Jiangsu) Co. Ltd

In September 2018, the Company acquired NSCC Carbon, a carbon black manufacturing facility in Pizhou, Jiangsu Province, China for a purchase price of $8 million. The purchase price is payable upon satisfaction of certain conditions that are expected to be satisfied within the 2019 calendar year and is recorded within Accounts payable and accrued liabilities on the Consolidated Balance Sheets. The Company plans to modify this facility to produce specialty carbons. The plant is temporarily mothballed. The modifications are expected to be completed, and production is expected to commence, in 2021. During the three and nine months ended June 30, 2019 the Company has incurred $1 million and $4 million, respectively, of transition-related costs related to this acquisition.

Other Acquisition

In June 2019, the Company acquired a customer list, intellectual property, and trade names from a leading masterbatch producer in Asia. The total purchase price of $3 million was allocated to customer relationships and is presented in Intangible assets, net on the Consolidated Balance Sheets. The acquisition extends the Company’s global footprint in black and white masterbatch in the Specialty Compounds business.

Sale of the Specialty Fluids Business

9 Months Ended

Jun. 30, 2019

Discontinued Operations And Disposal Groups [Abstract]

Sale of the Specialty Fluids Business

Note D. Sale of the Specialty Fluids Business

In June 2019, the Company completed the sale of its Specialty Fluids business, an operating segment of the Company, to Sinomine for total proceeds of $133 million, net of $2 million cash transferred, subject to customary post-closing adjustments that are not expected to be material. Five million dollars of the proceeds are being held in escrow, and are included in Prepaid expenses and other current assets on the Consolidated Balance Sheets, pending the issuance of a Certificate of Compliance by the Canada Revenue Agency related to withholding tax on taxable Canadian property.

The Company recognized a pre-tax loss on the sale of the Specialty Fluids business of $8 million in the third quarter of fiscal 2019 and a $20 million impairment charge during the second quarter of fiscal 2019. The sale of the Specialty Fluids business does not meet the criteria to be reported as a discontinued operation as it does not constitute a significant strategic business shift for the Company, and has no major effect on operations.

As part of the sale, the Company may receive additional cash royalties of up to $5 million for lithium products, payable over a ten-year period.

Employee Benefit Plans

9 Months Ended

Jun. 30, 2019

Compensation And Retirement Disclosure [Abstract]

Employee Benefit Plans

E. Employee Benefit Plans

Net periodic defined benefit pension and other postretirement benefit costs include the following:

	Three Months Ended June 30
	2019						2018						2019					2018
	Pension Benefits												Postretirement Benefits
	U.S.			Foreign			U.S.			Foreign			U.S.			Foreign		U.S.			Foreign
	(In millions)
Service cost	$	1		$	2		$	1		$	2		$	—		$	—	$	—		$	—
Interest cost		1			1			1			2			—			—		1			—
Expected return on plan assets		(2	)		(2	)		(3	)		(4	)		—			—		—			—
Amortization of prior service cost (credit)		—			2			—			—			—			—		(1	)		—
Amortization of actuarial loss		—			1			—			1			(1	)		—		—			—
Settlement and curtailment gain		—			—			—			—			—			—		—			—
Net periodic benefit (credit) cost	$	—		$	4		$	(1	)	$	1		$	(1	)	$	—	$	—		$	—

	Nine Months Ended June 30
	2019						2018						2019					2018
	Pension Benefits												Postretirement Benefits
	U.S.			Foreign			U.S.			Foreign			U.S.			Foreign		U.S.			Foreign
	(In millions)
Service cost	$	1		$	5		$	1		$	6		$	—		$	—	$	—		$	—
Interest cost		4			4			3			6			1			—		1			—
Expected return on plan assets		(7	)		(8	)		(8	)		(11	)		—			—		—			—
Amortization of prior service cost (credit)		—			2			—			—			(1	)		—		(2	)		—
Amortization of actuarial loss		—			2			—			2			(1	)		—		—			—
Settlement and curtailment gain		—			(6	)		—			—			—			—		—			—
Net periodic benefit (credit) cost	$	(2	)	$	(1	)	$	(4	)	$	3		$	(1	)	$	—	$	(1	)	$	—

In the third quarter of fiscal 2019, the Company adjusted the assumptions in its U.K. plan to calculate accrued benefits for a portion of the plan’s participants. As a result of this change, a prior service cost of $2 million was recorded in Other income (expense) in the Consolidated Statement of Operations.

Curtailments and Settlements of Employee Benefit Plans

In December 2018, the Company transferred the majority of the defined benefit obligations and pension plan assets in one of its foreign defined benefit plans to a multi-employer plan. This action moved the administrative, asset custodial, asset investment, actuarial, communication and benefit payment obligations to the multi-employer fund administrator. As a result of the transfer, there was a $30 million reduction in net pension obligations associated with the plan, and a pre-tax gain of $6 million was recorded in the first quarter of fiscal 2019, which is included in Other income (expense) in the Consolidated Statement of Operations. The pre-tax gain consisted of a curtailment credit of less than $1 million for the accelerated recognition of prior service credit and a settlement credit of $5 million for the accelerated recognition of the unrecognized gain upon the transfer of the assets. As of June 30, 2019, approximately $1 million of plan assets remain to be transferred. Such assets are expected to be transferred by the end of fiscal 2019 and such transfer may result in an additional immaterial settlement gain or loss. In addition, as part of the transfer the Company recorded a $3 million charge in the first quarter of fiscal 2019 for the Company’s agreement to fund the actuarial loss gap between the terminated plan and the multi-employer plan, which will be paid over the next five years. This charge is included Other income (expense) in the Consolidated Statement of Operations and the liability is included in Accounts payable and accrued liabilities and Other liabilities on the Consolidated Balance Sheet.

Purification Solutions Goodwill and Long-Lived Assets Impairment Charges

9 Months Ended

Jun. 30, 2019

Goodwill And Intangible Assets Disclosure [Abstract]

Purification Solutions Goodwill and Long-Lived Assets Impairment Charges

F. Purification Solutions Goodwill and Long-Lived Assets Impairment Charges

During the second quarter of fiscal 2018, the Company recorded impairment charges relating to the goodwill and long-lived assets of the Purification Solutions reporting unit, and an associated deferred tax benefit, in the Consolidated Statement of Operations as follows:

	Three Months Ended March 31, 2018
	(In millions)
Purification Solutions goodwill impairment charge	$	92
Purification Solutions long-lived assets impairment charge		162
Benefit for income taxes		(30	)
Impairment charges, after tax	$	224

In the second quarter of fiscal 2018, the Purification Solutions reporting unit had experienced market share losses, lower customer demand and declining prices in the mercury removal and North America powdered activated carbon applications, which led the Company to reassess its previous estimates for expected growth in volumes, prices and margins in the reporting unit. Due to the revised forecasts at the time, the Company performed a quantitative goodwill impairment test and determined that the estimated fair value of the Purification Solutions reporting unit was lower than the reporting unit's carrying value, resulting in a goodwill impairment charge of $92 million.

Prior to determining the goodwill impairment charge, the Company considered whether the assets of the reporting unit were recoverable. As a result of that assessment, the Company recorded an inventory reserve adjustment of $13 million and impairments to long-lived assets of $162 million. The adjustment to inventory carrying value was determined based on reassessments of volumes, pricing, and margins for the reporting unit at the time and was recorded in Cost of sales in the Consolidated Statements of Operations. The impairment analysis to assess if definite-lived intangible assets and property, plant and equipment were recoverable was based on the estimated undiscounted cash flows of the reporting unit, and these cash flows were not sufficient to recover the carrying value of the long-lived assets over their remaining useful lives. Accordingly, the Company recorded impairment charges of $64 million and $98 million to its definite-lived intangible assets and property, plant and equipment, respectively, in the quarter ended March 31, 2018 based on the lower of the carrying amount or fair value of the long-lived assets.

Cabot will continue to monitor for events or changes in business circumstances that may indicate that the remaining carrying value of the asset group may not be recoverable.

Additionally, the Company recorded a tax benefit related to the impairment charges of $30 million in the second quarter of fiscal 2018, which was subsequently reduced by $1 million after the impairment charges by tax jurisdictions were finalized.

Goodwill and Intangible Assets

9 Months Ended

Jun. 30, 2019

Goodwill And Intangible Assets Disclosure [Abstract]

Goodwill and Intangible Assets

G. Goodwill and Intangible Assets

The carrying amount of goodwill attributable to each reportable segment with goodwill balances and the changes in those balances during the nine month period ended June 30, 2019 are as follows:

	Reinforcement Materials			Performance Chemicals		Total
	(In millions)
Balance at September 30, 2018	$	52		$	41	$	93
Foreign currency impact		(1	)		—		(1	)
Balance at June 30, 2019	$	51		$	41	$	92

The following table provides information regarding the Company’s intangible assets:

	June 30, 2019							September 30, 2018
	Gross Carrying Value		Accumulated Amortization			Net Intangible Assets		Gross Carrying Value		Accumulated Amortization			Net Intangible Assets
	(In millions)
Intangible assets with finite lives
Developed technologies	$	51	$	(4	)	$	47	$	52	$	(2	)	$	50
Trademarks		8		—			8		8		—			8
Customer relationships		58		(13	)		45		51		(11	)		40
Total intangible assets	$	117	$	(17	)	$	100	$	111	$	(13	)	$	98

Intangible assets are amortized over their estimated useful lives, which range between twelve and twenty-five years, with a weighted average amortization period of approximately nineteen years. Amortization expense for both three month periods ended June 30, 2019 and 2018 was $1 million and is included in Cost of sales, Selling and administrative expenses, and research and technical expenses in the Consolidated Statements of Operations. Amortization expense for the nine month periods ended June 30, 2019 and 2018 was $4 million and $6 million, respectively, and is included in Cost of sales, Selling and administrative expenses, and research and technical expenses in the Consolidated Statements of Operations. Total amortization expense is estimated to be approximately $6 million each year for the next five fiscal years.

Accumulated Other Comprehensive Income (Loss)

9 Months Ended

Jun. 30, 2019

Stockholders Equity Note [Abstract]

Commitments And Contingencies Disclosure [Abstract]

H. Accumulated Other Comprehensive Income (Loss)

Comprehensive income combines net income (loss) and other comprehensive income items, which are reported as components of stockholders’ equity in the accompanying Consolidated Balance Sheets.

Changes in each component of AOCI, net of tax, were as follows:

	Currency Translation Adjustment			Unrealized Gains on Investments		Pension and Other Postretirement Benefit Liability Adjustments			Total
	(In millions)
Balance at September 30, 2018, attributable to Cabot Corporation	$	(266	)	$	1	$	(52	)	$	(317	)
Other comprehensive income (loss) before reclassifications		(24	)		—		30			6
Amounts reclassified from AOCI		(1	)		—		(8	)		(9	)
Less: Other comprehensive income (loss) attributable to noncontrolling interests		—			—		—			—
Balance at December 31, 2018, attributable to Cabot Corporation	$	(291	)	$	1	$	(30	)	$	(320	)
Other comprehensive income (loss) before reclassifications		8			—		1			9
Amounts reclassified from AOCI		(1	)		—		(1	)		(2	)
Less: Other comprehensive income (loss) attributable to noncontrolling interests		2			—		—			2
Balance at March 31, 2019, attributable to Cabot Corporation		(286	)		1		(30	)		(315	)
Other comprehensive income (loss) before reclassifications		1			—		(1	)		—
Amounts reclassified from AOCI		—			—		2			2
Less: Other comprehensive income (loss) attributable to noncontrolling interests		(2	)		—		—			(2	)
Balance at June 30, 2019, attributable to Cabot Corporation	$	(283	)	$	1	$	(29	)	$	(311	)

The amounts reclassified out of AOCI and into the Consolidated Statements of Operations in the three and nine months ended June 30, 2019 and 2018 were as follows:

	Affected Line Item in the Consolidated	Three Months Ended June 30						Nine Months Ended June 30
	Statements of Operations	2019			2018			2019			2018
		(In millions)
Derivatives: net investment hedges
(Gains) losses reclassified to interest expense	Interest expense	$	(1	)	$	(2	)	$	(4	)	$	(4	)
(Gains) losses excluded from effectiveness testing and amortized to interest expense	Interest expense	$	—		$	—		$	1		$	—
Pension and other postretirement benefit liability adjustment
Amortization of actuarial losses	Net Periodic Benefit Cost - see Note E for details		—			1			1			2
Amortization of prior service cost (credit)	Net Periodic Benefit Cost - see Note E for details		2			(1	)		1			(2	)
Settlement and curtailment gain	Net Periodic Benefit Cost - see Note E for details		—			—			(6	)		—
Total before tax			1			(2	)		(7	)		(4	)
Tax impact	Provision (benefit) for income taxes		1			1			(2	)		1
Total after tax		$	2		$	(1	)	$	(9	)	$	(3	)

Commitments and Contingencies

9 Months Ended

Jun. 30, 2019

Commitments and Contingencies

I. Commitments and Contingencies

Purchase Commitments

Cabot has entered into long-term purchase agreements primarily for the purchase of raw materials. Under certain of these agreements, the quantity of material being purchased is fixed, but the price paid changes as market prices change. For these purchase commitments, the amounts included in the table below are based on market prices at June 30, 2019, which may differ from actual market prices at the time of purchase.

	Payments Due by Fiscal Year
	Remainder of Fiscal 2019		2020		2021		2022		2023		Thereafter		Total
	(In millions)
Reinforcement Materials	$	100	$	251	$	139	$	131	$	123	$	1,584	$	2,328
Performance Chemicals		27		63		59		57		34		510		750
Purification Solutions		1		6		1		—		—		—		8
Total	$	128	$	320	$	199	$	188	$	157	$	2,094	$	3,086

Guarantee Agreements

Cabot has provided certain indemnities pursuant to which it may be required to make payments to an indemnified party in connection with certain transactions and agreements. In connection with certain acquisitions and divestitures, Cabot has provided routine indemnities with respect to such matters as environmental, tax, insurance, product and employee liabilities. In connection with various other agreements, including service and supply agreements with customers, Cabot has provided indemnities for certain contingencies and routine warranties. Cabot is unable to estimate the maximum potential liability for these types of indemnities as a maximum obligation is not explicitly stated in most cases and the amounts, if any, are dependent upon the outcome of future contingent events, the nature and likelihood of which cannot be reasonably estimated. The duration of the indemnities vary, and in many cases are indefinite. Cabot has not recorded any liability for these indemnities in the consolidated financial statements, except as otherwise disclosed.

Contingencies

Cabot is a defendant, or potentially responsible party, in various lawsuits and environmental proceedings wherein substantial amounts are claimed or at issue.

Environmental Matters

As of June 30, 2019 and September 30, 2018, Cabot had $14 million and $15 million, respectively, reserved for environmental matters. These environmental matters mainly relate to former operations. The Company’s reserves for environmental matters represent Cabot’s best estimates of the probable costs to be incurred at those sites where costs are reasonably estimable based on the Company’s analysis of the extent of clean up required, alternative clean-up methods available, abilities of other responsible parties to contribute and its interpretation of laws and regulations applicable to each site.

Cash payments related to these environmental matters were $1 million and $2 million in the first nine months of fiscal 2019 and fiscal 2018, respectively. Cabot reviews the adequacy of the reserves as circumstances change at individual sites and adjusts the reserves as appropriate. Almost all of Cabot’s environmental issues relate to sites that are mature and have been investigated and studied and, in many cases, are subject to agreed upon remediation plans. However, depending on the results of future testing, changes in risk assessment practices, remediation techniques and regulatory requirements, newly discovered conditions, and other factors, it is reasonably possible that the Company could incur additional costs in excess of environmental reserves currently recorded. Management estimates, based on the latest available information, that any such future environmental remediation costs that are reasonably possible to be in excess of amounts already recorded would be immaterial to the Company’s consolidated financial statements.

Respirator Liabilities

Cabot has exposure in connection with a safety respiratory products business that a subsidiary acquired from American Optical Corporation (“AO”) in an April 1990 asset purchase transaction. The subsidiary manufactured respirators under the AO brand and disposed of that business in July 1995. In connection with its acquisition of the business, the subsidiary agreed, in certain circumstances, to assume a portion of AO’s liabilities, including costs of legal fees together with amounts paid in settlements and judgments, allocable to AO respiratory products used prior to the 1990 purchase by the Cabot subsidiary. In exchange for the subsidiary’s assumption of certain of AO’s respirator liabilities, AO agreed to provide to the subsidiary the benefits of: (i) AO’s insurance coverage for the period prior to the 1990 acquisition and (ii) a former owner’s indemnity of AO holding it harmless from any liability allocable to AO respiratory products used prior to May 1982. As more fully described in the 2018 10-K, the respirator liabilities generally involve claims for personal injury, including asbestosis, silicosis and coal worker’s pneumoconiosis, allegedly resulting from the use of respirators that are alleged to have been negligently designed and/or labeled. Neither Cabot, nor its past or present subsidiaries, at any time manufactured asbestos or asbestos-containing products. At no time did this respiratory product line represent a significant portion of the respirator market.

As of June 30, 2019, and September 30, 2018, there were approximately 36,000 and 35,000 claimants, respectively, in pending cases asserting claims against AO in connection with respiratory products. Cabot has a reserve to cover its expected share of liability for existing and future respirator liability claims.

The Company’s current estimate of the cost of its share of existing and future respirator liability claims is based on facts and circumstances existing at this time. Developments that could affect the Company’s estimate include, but are not limited to, (i) significant changes in the number of future claims, (ii) changes in the rate of dismissals without payment of pending claims, (iii) significant changes in the average cost of resolving claims, including potential settlements of groups of claims, (iv) significant changes in the legal costs of defending these claims, (v) changes in the nature of claims received, (vi) trial and appellate outcomes, (vii) changes in the law and procedure applicable to these claims, (viii) the financial viability of the parties that contribute to the settlement of respirator claims, (ix) a change in the availability of insurance coverage maintained by certain of the parties that contribute to the settlement of respirator claims, or the indemnity provided by a former owner of the business, (x) changes in the allocation of costs among the various parties paying legal and settlement costs, and (xi) a determination that the assumptions that were used to estimate Cabot’s share of liability are no longer reasonable. The Company cannot determine the impact of these potential developments on its current estimate of its share of liability for existing and future claims. Accordingly, the actual amount of these liabilities for existing and future claims could be larger than the reserved amount.

At June 30, 2019 and September 30, 2018, the reserve to cover the Company’s expected share of liability for existing and future respirator liability claims was $21 million and $25 million, respectively. The Company made payments related to its respirator liability of $4 million and $1 million in the first nine months of fiscal 2019 and fiscal 2018, respectively.

Other Matters

The Company has various other lawsuits, claims and contingent liabilities arising in the ordinary course of its business and with respect to its divested businesses. The Company does not believe that any of these matters will have a material adverse effect on its financial position; however, litigation is inherently unpredictable. Cabot could incur judgments, enter into settlements or revise its expectations regarding the outcome of certain matters, and such developments could have a material impact on its results of operations in the period in which the amounts are accrued or its cash flows in the period in which the amounts are paid.

Income Tax

9 Months Ended

Jun. 30, 2019

Income Tax

J. Income Tax

Effective Tax Rate

	Three Months Ended June 30						Nine Months Ended June 30
	2019			2018			2019			2018
	(Dollars in millions)
(Provision) benefit for income taxes	$	(30	)	$	4		$	(43	)	$	(194	)
Effective tax rate		43	%		(3	)%		23	%		1202	%

For the three and nine months ended June 30, 2019, the tax (provision) benefit included a net discrete tax expense of $13 million and a net discrete tax benefit of $11 million, respectively. Of these amounts, a net tax expense of $17 million and zero, respectively, are related to U.S. tax reform legislation described below. For the three and nine months ended June 30, 2018, the tax (provision) benefit included a net discrete tax benefit of $23 million and a net discrete tax expense of $139 million, respectively. Of these amounts, a net tax benefit of $4 million and a net tax expense of $185 million, respectively, are related to U.S. tax reform legislation described below.

Tax Reform

On December 22, 2017, the U.S. enacted significant changes to federal income tax law affecting the Company, including a permanent reduction of the U.S. corporate income tax rate from 35% to 21%, effective January 1, 2018, as well as a 100% dividend received deduction for foreign dividends. Although the passage of the Act reduced the U.S. tax rate and effectively created a participation exemption regime for foreign earnings, in transitioning to this participation exemption regime, Cabot was also subject, during fiscal 2018, to a one-time tax on the deemed repatriation of certain foreign earnings (the “transition tax”).

As provided in Staff Accounting Bulletin 118 (“SAB 118”), the Company made certain final adjustments during the first quarter of fiscal 2019, including, in particular, the recording of a $17 million tax benefit related to the U.S. taxation of deemed foreign dividends in the transition fiscal year. During the quarter ended June 30, 2019, the U.S. Department of the Treasury issued final regulations denying fiscal year taxpayers the ability to claim a deduction for the gross-up portion of foreign dividends attributable to the transition tax. As a result, the Company reversed the original benefit resulting in an additional tax expense of $17 million in the third quarter of fiscal 2019 related to the U.S. taxation of deemed foreign dividends in the transition fiscal year.

Uncertainties

Cabot and certain subsidiaries are under audit in a number of jurisdictions. In addition, certain statutes of limitations are scheduled to expire in the near future. It is reasonably possible that a further change in the unrecognized tax benefits may also occur within the next twelve months related to the settlement of one or more of these audits or the lapse of applicable statutes of limitations. However, an estimated range of the impact on the unrecognized tax benefits cannot be quantified at this time.

Cabot files U.S. federal and state and non-U.S. income tax returns in jurisdictions with varying statutes of limitations. The 2014 through 2017 tax years generally remain subject to examination by the IRS and various tax years from 2005 through 2017 remain subject to examination by the respective state tax authorities. In significant non-U.S. jurisdictions, various tax years from 2002 through 2017 remain subject to examination by their respective tax authorities. As of June 30, 2019, Cabot’s significant non-U.S. jurisdictions include Canada, China, France, Germany, Italy, Japan, and the Netherlands.

During the three and nine months ended June 30, 2019, Cabot released uncertain tax positions of $1 million and $9 million, respectively, due to audit settlements and the expiration of statutes of limitations in various jurisdictions. During the three months ended June 30, 2018, Cabot did not release any uncertain tax positions. During the nine months ended June 30, 2018, Cabot released uncertain tax positions of $2 million due to the expiration of statutes of limitations in various jurisdictions.

Earnings Per Share

9 Months Ended

Jun. 30, 2019

Restructuring And Related Activities [Abstract]

Earnings Per Share

K. Earnings Per Share

The following tables summarize the components of the basic and diluted earnings (loss) per common share (“EPS”) computations:

	Three Months Ended June 30				Nine Months Ended June 30
	2019		2018		2019		2018
	(In millions, except per share amounts)
Basic EPS:
Net income (loss) attributable to Cabot Corporation	$	32	$	88	$	124	$	(207	)
Less: Undistributed earnings allocated to participating securities⁽¹⁾		—		—		—		—
Earnings (loss) allocated to common shareholders (numerator)	$	32	$	88	$	124	$	(207	)

Weighted average common shares and participating securities outstanding		59.0		62.5		59.9		62.5
Less: Participating securities⁽¹⁾		0.8		0.7		0.8		0.7
Adjusted weighted average common shares (denominator)		58.2		61.8		59.1		61.8

Earnings (loss) per common share - basic:	$	0.55	$	1.41	$	2.08	$	(3.36	)

Diluted EPS:
Earnings (loss) allocated to common shareholders	$	32	$	88	$	124	$	(207	)
Plus: Earnings (loss) allocated to participating securities		—		1		1		1
Less: Adjusted earnings allocated to participating securities⁽²⁾		—		1		1		1
Earnings (loss) allocated to common shareholders (numerator)	$	32	$	88	$	124	$	(207	)

Adjusted weighted average common shares outstanding		58.2		61.8		59.1		61.8
Effect of dilutive securities:
Common shares issuable⁽³⁾		0.2		0.5		0.1		—
Adjusted weighted average common shares (denominator)		58.4		62.3		59.2		61.8

Earnings (loss) per common share - diluted:	$	0.55	$	1.40	$	2.08	$	(3.36	)

⁽¹⁾	^{Participating securities consist of shares underlying outstanding and achieved performance-based restricted stock units issued during and after fiscal 2017 and all unvested time-based restricted stock units.}

Undistributed earnings are the earnings which remain after dividends declared during the period are assumed to be distributed to the common and participating shareholders. Undistributed earnings are allocated to common and participating shareholders on the same basis as dividend distributions. The calculation of undistributed earnings is as follows:

	Three Months Ended June 30				Nine Months Ended June 30
	2019		2018		2019		2018
	(In millions)
Calculation of undistributed earnings (loss):
Net income (loss) attributable to Cabot Corporation	$	32	$	88	$	124	$	(207	)
Less: Dividends declared on common stock		20		21		60		60
Undistributed earnings (loss)	$	12	$	67	$	64	$	(267	)

Allocation of undistributed earnings (loss):
Undistributed earnings (loss) allocated to common shareholders	$	12	$	67	$	64	$	(267	)
Undistributed earnings (loss)	$	12	$	67	$	64	$	(267	)

⁽²⁾	Undistributed earnings are adjusted for the assumed distribution of dividends to the dilutive securities, which are described in (3) below, and then reallocated to participating securities.

⁽³⁾

Represents incremental shares of common stock from the (i) assumed exercise of stock options issued under Cabot’s equity incentive plans; (ii) assumed issuance of shares to employees pursuant to the Company’s Deferred Compensation and Supplemental Retirement Plan; and (iii) assumed issuance of shares for outstanding and achieved performance-based restricted stock unit awards issued before fiscal 2017 under Cabot’s equity incentive plans using the treasury stock method. For the three and nine months ended June 30, 2019, 983,081 and 945,246 incremental shares of common stock, respectively, were excluded from the calculation of diluted earnings per share because the inclusion of these shares would have been antidilutive. For the three and nine months ended June 30, 2018, 260,630 and 799,163 incremental shares of common stock, respectively, were excluded from the calculation of diluted earnings per share because the inclusion of these shares would have been antidilutive.

Restructuring

9 Months Ended

Jun. 30, 2019

Restructuring

L. Restructuring

Cabot’s restructuring activities were recorded in the Consolidated Statements of Operations in the three and nine months ended June 30, 2019 and 2018 as follows:

	Three Months Ended June 30				Nine Months Ended June 30
	2019		2018		2019		2018
	(In millions)
Cost of sales	$	2	$	1	$	7	$	(8	)
Selling and administrative expenses		2		—		8		1
Total	$	4	$	1	$	15	$	(7	)

Details of all restructuring activities and the related reserves during the three and nine months ended June 30, 2019 were as follows:

	Severance and Employee Benefits			Environmental Remediation		Asset Impairment and Accelerated Depreciation			Other			Total
	(In millions)
Reserve at September 30, 2018	$	1		$	4	$	—		$	—		$	5
Charges (gain)		7			—		1			1			9
Costs charged against assets / (liabilities)		—			1		(1	)		—			—
Cash paid		(1	)		—		—			(1	)		(2	)
Reserve at December 31, 2018		7			5		—			—			12
Charges (gain)		1			—		—			1			2
Costs charged against assets / (liabilities)		—			—		—			—			—
Cash paid		(1	)		—		—			(1	)		(2	)
Reserve at March 31, 2019		7			5		—			—			12
Charges (gain)		2			—		1			1			4
Costs charged against assets / (liabilities)		—			—		(1	)					(1	)
Cash paid		(2	)		—		—			(1	)		(3	)
Reserve at June 30, 2019	$	7		$	5	$	—		$	—		$	12

Cabot’s severance and employee benefit reserves and other closure related reserves are reflected in Accounts payable and accrued liabilities on the Company’s Consolidated Balance Sheets. Cabot’s environmental remediation reserves related to restructuring activities are reflected in Other liabilities on the Company’s Consolidated Balance Sheets.

Purification Solutions Transformation Plan

In December 2018, the Company began implementation of a transformation plan to improve the long-term performance of the Purification Solutions segment. The purpose of the plan is to focus the business’s product portfolio, optimize its manufacturing assets, and streamline its organizational structure to support the new focus. The Company recorded charges of less than $1 million of severance costs related to this plan in the three months ended June 30, 2019 and recorded charges of approximately $9 million related to this plan in the nine months ended June 30, 2019, comprised of approximately $8 million of severance costs and $1 million of professional fees. The Company expects to record cash charges of approximately $1 million in the remainder of fiscal 2019 and thereafter for additional restructuring charges related to this plan. Cabot paid approximately $2 million and $4 million related to these activities in the three and nine months ended June 30, 2019, respectively, and expects to pay approximately $6 million in the remainder of fiscal 2019 and thereafter. As of June 30, 2019, Cabot had $5 million of accrued severance charges in the Consolidated Balance Sheets related to these actions.

2016 Plan

As part the Company’s 2016 restructuring plan, the Company ceased operations at its carbon black manufacturing facility in Merak, Indonesia in January 2016 and completed the sale of the land on which the facility was located in the second quarter of fiscal 2018 for cash consideration totaling approximately $13 million, resulting in a net pre-tax gain of approximately $11 million recorded to Cost of sales in the Company’s Consolidated Statements of Operations.

Financial Instruments and Fair Value Measurements

9 Months Ended

Jun. 30, 2019

Fair Value Disclosures [Abstract]

Financial Instruments and Fair Value Measurements

M. Financial Instruments and Fair Value Measurements

The FASB authoritative guidance on fair value measurements defines fair value, provides a framework for measuring fair value, and requires certain disclosures about fair value measurements. The required disclosures focus on the inputs used to measure fair value. The guidance establishes the following hierarchy for categorizing these inputs:

Level 1	—	Quoted market prices in active markets for identical assets or liabilities

Level 2	—	Significant other observable inputs (e.g., quoted prices for similar items in active markets, quoted prices for identical or similar items in markets that are not active, inputs other than quoted prices that are observable such as interest rate and yield curves, and market-corroborated inputs)

Level 3	—	Significant unobservable inputs

There were no transfers of financial assets or liabilities measured at fair value between Level 1 and Level 2 and there were no Level 3 investments during the first nine months of either fiscal 2019 or 2018.

At June 30, 2019 and September 30, 2018, Cabot had derivatives relating to foreign currency risks, including a net investment hedge and forward foreign currency contracts, carried at fair value. At June 30, 2019 and September 30, 2018, the fair value of these derivatives was a net liability of $8 million and $18 million, respectively, and was included in Prepaid expenses and other current assets and Other liabilities on the Consolidated Balance Sheets. These derivatives are classified as Level 2 instruments within the fair value hierarchy as the fair value determination was based on observable inputs.

At both June 30, 2019 and September 30, 2018, the fair value of guaranteed investment contracts, included in Other assets on the Consolidated Balance Sheets, was $11 million. Guaranteed investment contracts were classified as Level 2 instruments within the fair value hierarchy as the fair value determination was based on other observable inputs.

At June 30, 2019 and September 30, 2018, the fair values of cash and cash equivalents, accounts and notes receivable, accounts payable and accrued liabilities, and short-term borrowings and variable rate debt approximated their carrying values due to the short-term nature of these instruments. The carrying value and fair value of the long-term fixed rate debt were $1,018 million and $1,077 million, respectively, as of June 30, 2019, and $747 million and $742 million, respectively, as of September 30, 2018. The fair values of Cabot’s fixed rate long-term debt are estimated based on comparable quoted market prices at the respective period ends. The carrying amounts of Cabot’s floating rate long-term debt and capital lease obligations approximate their fair values. All such measurements are based on observable inputs and are classified as Level 2 within the fair value hierarchy. The valuation technique used is the discounted cash flow model.

In May 2019, several subsidiaries entered into a revolving credit agreement with a loan commitment not to exceed 300 million Euros. The amount available for borrowing under this revolving credit agreement was $294 million as of June 30, 2019. The revolving credit agreement, which matures on May 22, 2024 or earlier upon maturity of the Company’s $1 billion unsecured revolving credit agreement, may be used for repatriation of earnings of Cabot’s foreign subsidiaries to the U.S., the repayment of indebtedness of the Company’s foreign subsidiaries owing to the Company or any of its subsidiaries, and for working capital and general corporate purposes. The obligations of the subsidiaries under the revolving credit agreement are guaranteed by the Company. The Company paid debt issuance costs of $1 million upon entering the agreement, which are being amortized over the life of the revolver.

In June 2019, Cabot issued $300 million in registered, unsecured, notes with a coupon of 4% that mature on July 1, 2029. Interest is payable under the notes semi-annually on January 1 and July 1 commencing in 2020. The net proceeds of this offering were $296 million after deducting discounts and issuance costs of $1 million and $3 million, respectively, which were paid at issuance and are being amortized over the life of the notes.

Derivatives

9 Months Ended

Jun. 30, 2019

Derivative Instruments And Hedging Activities Disclosure [Abstract]

Derivatives

N. Derivatives

Foreign Currency Risk Management

Cabot’s international operations are subject to certain risks, including currency exchange rate fluctuations and government actions. Cabot endeavors to match the currency in which debt is issued to the currency of the Company’s major, stable cash receipts. In some situations, Cabot has issued debt denominated in U.S. dollars and then entered into cross-currency swaps that exchange the dollar principal and interest payments into Euro-denominated principal and interest payments.

Additionally, the Company has foreign currency exposure arising from its net investments in foreign operations. Cabot may enter into cross-currency swaps to mitigate the impact of currency rate changes on the Company’s net investments.

The Company also has foreign currency exposure arising from the denomination of monetary assets and liabilities in foreign currencies other than the functional currency of a given subsidiary as well as the risk that currency fluctuations could affect the dollar value of future cash flows generated in foreign currencies. Accordingly, Cabot uses short-term forward contracts to minimize the exposure to foreign currency risk. In certain situations where the Company has forecasted purchases under a long-term commitment or forecasted sales denominated in a foreign currency, Cabot may enter into appropriate financial instruments in accordance with the Company’s risk management policy to hedge future cash flow exposures.

The following table provides details of the derivatives held as of June 30, 2019 and September 30, 2018 to manage foreign currency risk.

		Notional Amount
Description	Borrowing	June 30, 2019	September 30, 2018	Hedge Designation
Cross-Currency Swaps	3.4% Notes	USD 250 million swapped to EUR 223 million	USD 250 million swapped to EUR 223 million	Net investment
Forward Foreign Currency Contracts ⁽¹⁾	N/A	USD 28 million	USD 18 million	No designation

⁽¹⁾	Cabot’s forward foreign exchange contracts are denominated in the Canadian dollar, Indonesian rupiah and Czech koruna.

Accounting for Derivative Instruments and Hedging Activities

The Company determines the fair value of financial instruments using quoted market prices whenever available. When quoted market prices are not available for various types of financial instruments (such as forwards, options and swaps), the Company uses standard models with market-based inputs, which take into account the present value of estimated future cash flows and the ability of Cabot or the financial counterparty to perform. For interest rate and cross-currency swaps, the significant inputs to these models are interest rate curves for discounting future cash flows and are adjusted for credit risk. For forward foreign currency contracts, the significant inputs are interest rate curves for discounting future cash flows, and exchange rate curves of the foreign currency for translating future cash flows.

Fair Value Hedge

For derivative instruments that are designated and qualify as fair value hedges, the gain or loss on the derivative as well as the offsetting gain or loss on the hedged item attributable to the hedged risk are recognized in current period earnings.

Cash Flow Hedge

For derivative instruments that are designated and qualify as cash flow hedges, the effective portion of the gain or loss on the derivative is recorded in AOCI and reclassified to earnings in the same period or periods during which the hedged transaction affects earnings. Gains and losses on the derivative representing either hedge ineffectiveness or hedge components excluded from the assessment of effectiveness are recognized in current period earnings.

Net Investment Hedge

For net investment hedges, changes in the fair value of the effective portion of the derivatives’ gains or losses are reported as foreign currency translation gains or losses in AOCI while changes in the ineffective portion are reported in earnings. Effectiveness is assessed using the method based on changes in spot exchange rates. The gains or losses on derivative instruments reported in AOCI are reclassified to earnings in the period in which earnings are affected by the underlying item, such as a disposal or substantial liquidations of the entities being hedged.

The Company has cross-currency swaps with a notional amount of $250 million, which are designated as hedges of its net investments in certain Euro-denominated subsidiaries. Cash settlements occur semi-annually on March 15^th and September 15^th for fixed rate interest payments and a cash exchange of the notional currency amount will occur at the end of the term in 2026. As of June 30, 2019, the fair value of these swaps was a net liability of $8 million and was included in Prepaid expenses and other current assets and Other liabilities and the cumulative loss of $5 million was included in AOCI on the Consolidated Balance Sheets. As of September 30, 2018, the fair value of these swaps was a net liability of $18 million and was included in Prepaid expenses and other current assets and Other Liabilities and the cumulative loss of $14 million was included in AOCI on the Consolidated Balance Sheets.

The following tables summarize the impact of the cross-currency swaps to AOCI and the Consolidated Statements of Operations:

	Three Months Ended June 30
	2019			2018		2019			2018			2019		2018
Description	Gain/(Loss) Recognized in AOCI					(Gain)/Loss Reclassified from AOCI into Interest Expense in the Consolidated Statements of Operations						(Gain)/Loss Recognized in Interest Expense in the Consolidated Statements of Operations (Amount Excluded from Effectiveness Testing)
	(In millions)
Cross-currency swaps	$	(1	)	$	10	$	(1	)	$	(2	)	$	—	$	—

	Nine Months Ended June 30
	2019		2018			2019			2018			2019		2018
Description	Gain/(Loss) Recognized in AOCI					(Gain)/Loss Reclassified from AOCI into Interest Expense in the Consolidated Statements of Operations						(Gain)/Loss Recognized in Interest Expense in the Consolidated Statements of Operations (Amount Excluded from Effectiveness Testing)
	(In millions)
Cross-currency swaps	$	12	$	(3	)	$	(4	)	$	(4	)	$	1	$	1

Other Derivative Instruments

From time to time, the Company may enter into certain derivative instruments that may not be designated as hedges for accounting purposes, which may include cross-currency swaps, foreign currency forward contracts and commodity derivatives. For cross-currency swaps and foreign currency forward contracts not designated as hedges, the Company uses standard models with market-based inputs. The significant inputs to these models are interest rate curves for discounting future cash flows, and exchange rate curves of the foreign currency for translating future cash flows. In determining the fair value of the commodity derivatives, the significant inputs to valuation models are quoted market prices of similar instruments in active markets. Although these derivatives do not qualify for hedge accounting, Cabot believes that such instruments are closely correlated with the underlying exposure, thus managing the associated risk. The gains or losses from changes in the fair value of derivative instruments that are not accounted for as hedges are recognized in current period earnings.

At June 30, 2019, the fair value of derivative instruments not designated as hedges were immaterial and were presented in Accounts payable and accrued liabilities, and Prepaid expenses and other current assets on the Consolidated Balance Sheets. At September 30, 2018, the fair value of derivative instruments not designated as hedges were immaterial and were presented in Accounts payable and accrued liabilities on the Consolidated Balance Sheets.

Hyperinflationary Economies

9 Months Ended

Jun. 30, 2019

Equity Method Investments And Joint Ventures [Abstract]

Hyperinflationary Economies

O. Hyperinflationary Economies

Argentina

Cabot owns 100% of a carbon black operating entity in Argentina. Effective July 1, 2018, the operating entity was considered to be functioning in a highly inflationary economy and began using Cabot’s reporting currency, the U.S. dollar, as its functional currency. Since the conversion, all impacts of foreign exchange changes between the reporting currency and Argentine peso are reflected in earnings in the Consolidated Statements of Operations.

The Company’s income from operations has not been significantly impacted from this change because the operating entity’s sales and a portion of its raw material purchases were already denominated in U.S. dollars. For the three and nine months ended June 30, 2019, the operating entity’s net revenue represented approximately 3% and 2%, respectively, of Cabot’s total net revenue.

The operating entity’s assets and liabilities held in local currency, which consist primarily of cash and cash equivalents, inventories, property, plant and equipment and accounts payable and accrued liabilities, made up less than 2% of Cabot’s total assets and total liabilities as of both June 30, 2019 and September 30, 2018. Changes in the Argentine peso exchange rate will result in foreign currency exchange gains or losses on the operating entity’s peso-denominated monetary assets and liabilities. For the three and nine months ended June 30, 2019, the Company recorded a net gain of less than $1 million and a net loss of $1 million, respectively, within Other (income) expense in the Consolidated Statements of Operations, which reflects the remeasurement of the operating entity’s net monetary liabilities denominated in Argentine pesos using an exchange rate of 42.7 Argentine pesos to the U.S. dollar at June 30, 2019.

The Company will continue to monitor the developments in Argentina and their potential impact on the operating entity’s operations or carrying value.

Venezuela

Cabot owns 49% of a carbon black operating affiliate in Venezuela, which is accounted for as an equity affiliate, through wholly-owned subsidiaries that carry the investment and receive its dividends. While the operating entity has historically been profitable, it has not been operational in recent periods due to a lack of available raw materials, which is expected to continue for the foreseeable future. As such, in the second quarter of fiscal 2019, the Company performed an impairment analysis and determined that the decrease in fair value of the Venezuelan equity investment is other-than-temporary and that the investment is fully impaired. The Company recorded an impairment charge of $11 million in the second quarter of fiscal 2019, which is included in Other income (expense) within the Consolidated Statement of Operations.

Financial Information by Segment

9 Months Ended

Jun. 30, 2019

Financial Information by Segment

P. Financial Information by Segment

The Company identifies a business as an operating segment if: (i) it engages in business activities from which it may earn revenues and incur expenses; (ii) its operating results are regularly reviewed by the Chief Operating Decision Maker (“CODM”), who is Cabot’s President and Chief Executive Officer, to make decisions about resources to be allocated to the segment and assess its performance; and (iii) it has available discrete financial information. The Company has determined that all of its businesses are operating segments. The CODM reviews financial information at the operating segment level to allocate resources and to assess the operating results and financial performance for each operating segment. Operating segments are aggregated into a reportable segment if the operating segments are determined to have similar economic characteristics and if the operating segments are similar in the following areas: (i) nature of products and services; (ii) nature of production processes; (iii) type or class of customer for their products and services; (iv) methods used to distribute the products or provide services; and (v) if applicable, the nature of the regulatory environment.

During the third quarter of fiscal 2019, the Company had four reportable segments: Reinforcement Materials, Performance Chemicals, Purification Solutions and Specialty Fluids. The Specialty Fluids business was divested as of June 28, 2019 and since that time Cabot has been organized into the three remaining reportable business segments.

The Reinforcement Materials segment consists of the rubber blacks and elastomer composites product lines.

The Performance Chemicals segment combines the specialty carbons, fumed metal oxides and aerogel product lines into the Performance Additives business, and combines the specialty compounds and inkjet colorants product lines into the Formulated Solutions business. These businesses are similar in terms of economic characteristics, nature of products, processes, customer class and product distribution methods, and, therefore, have been aggregated into one reportable segment. The net sales from each of these businesses for the three and nine months ended June 30, 2019 and 2018 were as follows:

	Three Months Ended June 30				Nine Months Ended June 30
	2019		2018		2019		2018
	(In millions)
Performance Additives	$	172	$	188	$	518	$	524
Formulated Solutions		79		86		218		247
Performance Chemicals	$	251	$	274	$	736	$	771

The Purification Solutions segment consists of the Company’s activated carbon business, and the Specialty Fluids segment included the cesium formate oil and gas drilling fluids and high-purity fine cesium chemicals product lines.

Income (loss) from continuing operations before income taxes (“Segment EBIT”) is presented for each reportable segment in the table below. Segment EBIT excludes certain items, meaning items management does not consider representative of on-going operating segment results. In addition, Segment EBIT includes Equity in earnings of affiliated companies, net of tax, the full operating results of a contractual joint venture in Purification Solutions, royalties, Net income attributable to noncontrolling interests, net of tax, and discounting charges for certain Notes receivable, but excludes Interest expense, foreign currency transaction gains and losses, interest income, dividend income, unearned revenue, general unallocated expense and unallocated corporate costs.

Financial information by reportable segment is as follows:

	Reinforcement Materials		Performance Chemicals		Purification Solutions			Specialty Fluids			Segment Total		Unallocated and Other⁽¹⁾			Consolidated Total
	(In millions)
Three Months Ended June 30, 2019
Revenues from external customers⁽²⁾	$	461	$	251	$	73		$	13		$	798	$	47		$	845
Income (loss) from continuing operations before income taxes⁽³⁾	$	72	$	37	$	1		$	2		$	112	$	(43	)	$	69

Three Months Ended June 30, 2018
Revenues from external customers⁽²⁾	$	466	$	274	$	70		$	12		$	822	$	32		$	854
Income (loss) from continuing operations before income taxes⁽³⁾	$	74	$	56	$	(6	)	$	3		$	127	$	(32	)	$	95

Nine Months Ended June 30, 2019
Revenues from external customers⁽²⁾	$	1,363	$	736	$	210		$	56		$	2,365	$	145		$	2,510
Income (loss) from continuing operations before income taxes⁽³⁾	$	195	$	111	$	(1	)	$	24		$	329	$	(141	)	$	188

Nine Months Ended June 30, 2018
Revenues from external customers⁽²⁾	$	1,307	$	771	$	206		$	24		$	2,308	$	84		$	2,392
Income (loss) from continuing operations before income taxes⁽³⁾	$	215	$	160	$	(6	)	$	(2	)	$	367	$	(351	)	$	16

⁽¹⁾	Unallocated and Other includes certain items and eliminations necessary to reflect management’s reporting of operating segment results. These items are reflective of the segment reporting presented to the CODM.

⁽²⁾

Consolidated Total Revenues from external customers reconciles to Net sales and other operating revenues on the Consolidated Statements of Operations. Revenues from external customers that are categorized as Unallocated and Other reflects royalties, external shipping and handling fees, the impact of unearned revenue, the removal of 100% of the sales of an equity method affiliate, discounting charges for certain Notes receivable, and by-product revenue. Details are provided in the table below:

	Three Months Ended June 30						Nine Months Ended June 30
	2019			2018			2019			2018
	(In millions)
Royalties, the impact of unearned revenue, the removal of 100% of the sales of an equity method affiliate and discounting charges for certain notes receivable	$	(3	)	$	(2	)	$	(9	)	$	(10	)
Shipping and handling fees		32			34			96			94
By-product sales^(a)		18			—			58			—
Total	$	47		$	32		$	145		$	84

^(a)

As of October 1, 2018, as part of the adoption of the new accounting standard, Revenue from Contracts with Customers, the Company began presenting revenue from by-products produced in manufacturing operations in Net sales and other operating revenues, which in prior years was included as a reduction in Cost of sales.

⁽³⁾

Consolidated Total Income (loss) from continuing operations before income taxes reconciles to Income (loss) from continuing operations before income taxes and equity in earnings of affiliated companies on the Consolidated Statements of Operations. Income (loss) from continuing operations before income taxes that are categorized as Unallocated and Other includes:

	Three Months Ended June 30						Nine Months Ended June 30
	2019			2018			2019			2018
	(In millions)
Interest expense	$	(14	)	$	(14	)	$	(43	)	$	(41	)
Certain items^(a)
Specialty Fluids loss on sale and asset impairment charge (Note D)		(8	)		—			(28	)		—
Equity affiliate investment impairment charge (Note O)		—			—			(11	)		—
Purification Solutions goodwill and long-lived assets impairment charge (Note F)		—			—			—			(254	)
Inventory reserve adjustment (Note F)		—			—			—			(13	)
Global restructuring activities (Note L)		(4	)		(1	)		(15	)		7
Legal and environmental matters and reserves		—			—			(1	)		(6	)
Gains (losses) on sale of investments		—			—			—			10
Acquisition and integration-related charges		(1	)		—			(5	)		(1	)
Other		(1	)		(2	)		(1	)		(3	)
Total certain items, pre-tax		(14	)		(3	)		(61	)		(260	)
Unallocated corporate costs^(b)		(14	)		(15	)		(39	)		(45	)
General unallocated income (expense)^(c)		—			—			3			(3	)
Less: Equity in earnings of affiliated companies, net of tax^(d)		1			—			1			2
Total	$	(43	)	$	(32	)	$	(141	)	$	(351	)

^(a)

Certain items are items of expense and income that management does not consider representative of the Company’s fundamental on-going segment results and they are, therefore, excluded from Segment EBIT.

^(b)

Unallocated corporate costs are costs that are not controlled by the segments and primarily benefit corporate interests.

^(c)

General unallocated income (expense) consists of gains (losses) arising from foreign currency transactions, net of other foreign currency risk management activities, the profit or loss related to the corporate adjustment for unearned revenue, and the impact of including the full operating results of a contractual joint venture in Purification Solutions Segment EBIT.

^(d)

Equity in earnings of affiliated companies, net of tax, is included in Segment EBIT and is removed in Unallocated and other to reconcile to Income (loss) from operations before income taxes and equity in earnings from affiliated companies.

The Company’s segments operate globally. In addition to presenting Revenue from external customers by reportable segment, the following tables further disaggregate Revenue from external customers by geographic region.

	Three Months Ended June 30, 2019
	Reinforcement Materials		Performance Chemicals		Purification Solutions		Specialty Fluids		Consolidated Total
	(In millions)
Americas	$	179	$	69	$	33	$	2	$	283
Asia Pacific		191		93		10		—		294
Europe, Middle East and Africa		91		89		30		11		221
Segment revenues from external customers		461		251		73		13		798
Unallocated and other										47
Net sales and other operating revenues									$	845

	Nine Months Ended June 30, 2019
	Reinforcement Materials		Performance Chemicals		Purification Solutions		Specialty Fluids		Consolidated Total
	(In millions)
Americas	$	515	$	222	$	93	$	6	$	836
Asia Pacific		573		253		26		1		853
Europe, Middle East and Africa		275		261		91		49		676
Segment revenues from external customers		1,363		736		210		56		2,365
Unallocated and other										145
Net sales and other operating revenues									$	2,510

Significant Accounting Policies (Policies)

9 Months Ended

Jun. 30, 2019

Intangible Assets and Goodwill Impairment

Revenue Recognition

period incurred and included in Cost of sales within the Consolidated Statement of Operations. Taxes collected on sales to customers are excluded from the transaction price.

The Company does not have contract assets or liabilities that are material.

Long-Lived Assets Impairment

Long-lived Assets Impairment

Property, Plant and Equipment

Income Tax in Interim Periods

Inventory Valuation

Pensions and Other Postretirement Benefits

Redeemable Preferred Stock

Recently Adopted Accounting Standards

Consolidated Statements of Operations	Three Months Ended June 30								Nine Months Ended June 30
	2018								2018
	As Originally Reported			Effect of Change			As Adjusted		As Originally Reported		Effect of Change			As Adjusted
	(In millions, except per share amounts)
Cost of sales	$	654		$	3		$	657	$	1,824	$	7		$	1,831
Gross profit	$	200		$	(3	)	$	197	$	568	$	(7	)	$	561
Selling and administrative expenses	$	74		$	—		$	74	$	221	$	2		$	223
Research and technical expenses	$	17		$	—		$	17	$	48	$	—		$	48
Income (loss) from operations	$	109		$	(3	)	$	106	$	45	$	(9	)	$	36
Other income (expense)	$	(2	)	$	3		$	1	$	4	$	9		$	13

Recent Accounting Pronouncements

Significant Accounting Policies (Tables)

9 Months Ended

Jun. 30, 2019

Schedule of Change Retrospectively to Prior Periods Financial Statement Line Items

The Company adopted the standard on October 1, 2018 and used a retrospective transition method to reclassify net periodic benefit cost, other than the service component, from Cost of sales, Selling and administrative expenses and Research and technical expenses to Other income (expense) in the Consolidated Statements of Operations for the prior periods presented. In accordance with the standard, the Company utilized prior period footnote disclosures as a practical expedient to apply these retrospective presentations, which is shown in the table below:

Consolidated Statements of Operations	Three Months Ended June 30								Nine Months Ended June 30
	2018								2018
	As Originally Reported			Effect of Change			As Adjusted		As Originally Reported		Effect of Change			As Adjusted
	(In millions, except per share amounts)
Cost of sales	$	654		$	3		$	657	$	1,824	$	7		$	1,831
Gross profit	$	200		$	(3	)	$	197	$	568	$	(7	)	$	561
Selling and administrative expenses	$	74		$	—		$	74	$	221	$	2		$	223
Research and technical expenses	$	17		$	—		$	17	$	48	$	—		$	48
Income (loss) from operations	$	109		$	(3	)	$	106	$	45	$	(9	)	$	36
Other income (expense)	$	(2	)	$	3		$	1	$	4	$	9		$	13

Employee Benefit Plans (Tables)

9 Months Ended

Jun. 30, 2019

Compensation And Retirement Disclosure [Abstract]

Net Periodic Defined Benefit Pension and Other Postretirement Benefit Costs

Net periodic defined benefit pension and other postretirement benefit costs include the following:

	Three Months Ended June 30
	2019						2018						2019					2018
	Pension Benefits												Postretirement Benefits
	U.S.			Foreign			U.S.			Foreign			U.S.			Foreign		U.S.			Foreign
	(In millions)
Service cost	$	1		$	2		$	1		$	2		$	—		$	—	$	—		$	—
Interest cost		1			1			1			2			—			—		1			—
Expected return on plan assets		(2	)		(2	)		(3	)		(4	)		—			—		—			—
Amortization of prior service cost (credit)		—			2			—			—			—			—		(1	)		—
Amortization of actuarial loss		—			1			—			1			(1	)		—		—			—
Settlement and curtailment gain		—			—			—			—			—			—		—			—
Net periodic benefit (credit) cost	$	—		$	4		$	(1	)	$	1		$	(1	)	$	—	$	—		$	—

	Nine Months Ended June 30
	2019						2018						2019					2018
	Pension Benefits												Postretirement Benefits
	U.S.			Foreign			U.S.			Foreign			U.S.			Foreign		U.S.			Foreign
	(In millions)
Service cost	$	1		$	5		$	1		$	6		$	—		$	—	$	—		$	—
Interest cost		4			4			3			6			1			—		1			—
Expected return on plan assets		(7	)		(8	)		(8	)		(11	)		—			—		—			—
Amortization of prior service cost (credit)		—			2			—			—			(1	)		—		(2	)		—
Amortization of actuarial loss		—			2			—			2			(1	)		—		—			—
Settlement and curtailment gain		—			(6	)		—			—			—			—		—			—
Net periodic benefit (credit) cost	$	(2	)	$	(1	)	$	(4	)	$	3		$	(1	)	$	—	$	(1	)	$	—

Purification Solutions Goodwill and Long-Lived Assets Impairment Charges (Tables)

9 Months Ended

Jun. 30, 2019

Goodwill And Intangible Assets Disclosure [Abstract]

Impairment Charges and Associated Deferred Tax Benefit

	Three Months Ended March 31, 2018
	(In millions)
Purification Solutions goodwill impairment charge	$	92
Purification Solutions long-lived assets impairment charge		162
Benefit for income taxes		(30	)
Impairment charges, after tax	$	224

Goodwill and Intangible Assets (Tables)

9 Months Ended

Jun. 30, 2019

Goodwill And Intangible Assets Disclosure [Abstract]

Schedule of Goodwill Balances

The carrying amount of goodwill attributable to each reportable segment with goodwill balances and the changes in those balances during the nine month period ended June 30, 2019 are as follows:

	Reinforcement Materials			Performance Chemicals		Total
	(In millions)
Balance at September 30, 2018	$	52		$	41	$	93
Foreign currency impact		(1	)		—		(1	)
Balance at June 30, 2019	$	51		$	41	$	92

Schedule of Intangible Assets

The following table provides information regarding the Company’s intangible assets:

	June 30, 2019							September 30, 2018
	Gross Carrying Value		Accumulated Amortization			Net Intangible Assets		Gross Carrying Value		Accumulated Amortization			Net Intangible Assets
	(In millions)
Intangible assets with finite lives
Developed technologies	$	51	$	(4	)	$	47	$	52	$	(2	)	$	50
Trademarks		8		—			8		8		—			8
Customer relationships		58		(13	)		45		51		(11	)		40
Total intangible assets	$	117	$	(17	)	$	100	$	111	$	(13	)	$	98

Accumulated Other Comprehensive Income (Loss) (Tables)

9 Months Ended

Jun. 30, 2019

Equity [Abstract]

Changes in Each Component of AOCI, Net of Tax

Changes in each component of AOCI, net of tax, were as follows:

	Currency Translation Adjustment			Unrealized Gains on Investments		Pension and Other Postretirement Benefit Liability Adjustments			Total
	(In millions)
Balance at September 30, 2018, attributable to Cabot Corporation	$	(266	)	$	1	$	(52	)	$	(317	)
Other comprehensive income (loss) before reclassifications		(24	)		—		30			6
Amounts reclassified from AOCI		(1	)		—		(8	)		(9	)
Less: Other comprehensive income (loss) attributable to noncontrolling interests		—			—		—			—
Balance at December 31, 2018, attributable to Cabot Corporation	$	(291	)	$	1	$	(30	)	$	(320	)
Other comprehensive income (loss) before reclassifications		8			—		1			9
Amounts reclassified from AOCI		(1	)		—		(1	)		(2	)
Less: Other comprehensive income (loss) attributable to noncontrolling interests		2			—		—			2
Balance at March 31, 2019, attributable to Cabot Corporation		(286	)		1		(30	)		(315	)
Other comprehensive income (loss) before reclassifications		1			—		(1	)		—
Amounts reclassified from AOCI		—			—		2			2
Less: Other comprehensive income (loss) attributable to noncontrolling interests		(2	)		—		—			(2	)
Balance at June 30, 2019, attributable to Cabot Corporation	$	(283	)	$	1	$	(29	)	$	(311	)

Amounts Reclassified Out of AOCI

The amounts reclassified out of AOCI and into the Consolidated Statements of Operations in the three and nine months ended June 30, 2019 and 2018 were as follows:

	Affected Line Item in the Consolidated	Three Months Ended June 30						Nine Months Ended June 30
	Statements of Operations	2019			2018			2019			2018
		(In millions)
Derivatives: net investment hedges
(Gains) losses reclassified to interest expense	Interest expense	$	(1	)	$	(2	)	$	(4	)	$	(4	)
(Gains) losses excluded from effectiveness testing and amortized to interest expense	Interest expense	$	—		$	—		$	1		$	—
Pension and other postretirement benefit liability adjustment
Amortization of actuarial losses	Net Periodic Benefit Cost - see Note E for details		—			1			1			2
Amortization of prior service cost (credit)	Net Periodic Benefit Cost - see Note E for details		2			(1	)		1			(2	)
Settlement and curtailment gain	Net Periodic Benefit Cost - see Note E for details		—			—			(6	)		—
Total before tax			1			(2	)		(7	)		(4	)
Tax impact	Provision (benefit) for income taxes		1			1			(2	)		1
Total after tax		$	2		$	(1	)	$	(9	)	$	(3	)

Commitments and Contingencies (Tables)

9 Months Ended

Jun. 30, 2019

Commitments And Contingencies Disclosure [Abstract]

Schedule of Components of Purchase Commitments

	Payments Due by Fiscal Year
	Remainder of Fiscal 2019		2020		2021		2022		2023		Thereafter		Total
	(In millions)
Reinforcement Materials	$	100	$	251	$	139	$	131	$	123	$	1,584	$	2,328
Performance Chemicals		27		63		59		57		34		510		750
Purification Solutions		1		6		1		—		—		—		8
Total	$	128	$	320	$	199	$	188	$	157	$	2,094	$	3,086

Income Tax (Tables)

9 Months Ended

Jun. 30, 2019

Effective Tax Rate

	Three Months Ended June 30						Nine Months Ended June 30
	2019			2018			2019			2018
	(Dollars in millions)
(Provision) benefit for income taxes	$	(30	)	$	4		$	(43	)	$	(194	)
Effective tax rate		43	%		(3	)%		23	%		1202	%

Earnings Per Share (Tables)

9 Months Ended

Jun. 30, 2019

Components of Basic and Diluted Earnings (Loss) Per Common Share (EPS)

The following tables summarize the components of the basic and diluted earnings (loss) per common share (“EPS”) computations:

	Three Months Ended June 30				Nine Months Ended June 30
	2019		2018		2019		2018
	(In millions, except per share amounts)
Basic EPS:
Net income (loss) attributable to Cabot Corporation	$	32	$	88	$	124	$	(207	)
Less: Undistributed earnings allocated to participating securities⁽¹⁾		—		—		—		—
Earnings (loss) allocated to common shareholders (numerator)	$	32	$	88	$	124	$	(207	)

Weighted average common shares and participating securities outstanding		59.0		62.5		59.9		62.5
Less: Participating securities⁽¹⁾		0.8		0.7		0.8		0.7
Adjusted weighted average common shares (denominator)		58.2		61.8		59.1		61.8

Earnings (loss) per common share - basic:	$	0.55	$	1.41	$	2.08	$	(3.36	)

Diluted EPS:
Earnings (loss) allocated to common shareholders	$	32	$	88	$	124	$	(207	)
Plus: Earnings (loss) allocated to participating securities		—		1		1		1
Less: Adjusted earnings allocated to participating securities⁽²⁾		—		1		1		1
Earnings (loss) allocated to common shareholders (numerator)	$	32	$	88	$	124	$	(207	)

Adjusted weighted average common shares outstanding		58.2		61.8		59.1		61.8
Effect of dilutive securities:
Common shares issuable⁽³⁾		0.2		0.5		0.1		—
Adjusted weighted average common shares (denominator)		58.4		62.3		59.2		61.8

Earnings (loss) per common share - diluted:	$	0.55	$	1.40	$	2.08	$	(3.36	)

⁽¹⁾	^{Participating securities consist of shares underlying outstanding and achieved performance-based restricted stock units issued during and after fiscal 2017 and all unvested time-based restricted stock units.}

Calculation of Undistributed Earnings

	Three Months Ended June 30				Nine Months Ended June 30
	2019		2018		2019		2018
	(In millions)
Calculation of undistributed earnings (loss):
Net income (loss) attributable to Cabot Corporation	$	32	$	88	$	124	$	(207	)
Less: Dividends declared on common stock		20		21		60		60
Undistributed earnings (loss)	$	12	$	67	$	64	$	(267	)

Allocation of undistributed earnings (loss):
Undistributed earnings (loss) allocated to common shareholders	$	12	$	67	$	64	$	(267	)
Undistributed earnings (loss)	$	12	$	67	$	64	$	(267	)

⁽²⁾	Undistributed earnings are adjusted for the assumed distribution of dividends to the dilutive securities, which are described in (3) below, and then reallocated to participating securities.

⁽³⁾

Restructuring (Tables)

9 Months Ended

Jun. 30, 2019

Restructuring And Related Activities [Abstract]

Recorded Restructuring Activities

Cabot’s restructuring activities were recorded in the Consolidated Statements of Operations in the three and nine months ended June 30, 2019 and 2018 as follows:

	Three Months Ended June 30				Nine Months Ended June 30
	2019		2018		2019		2018
	(In millions)
Cost of sales	$	2	$	1	$	7	$	(8	)
Selling and administrative expenses		2		—		8		1
Total	$	4	$	1	$	15	$	(7	)

Restructuring Activities and Related Reserves

Details of all restructuring activities and the related reserves during the three and nine months ended June 30, 2019 were as follows:

	Severance and Employee Benefits			Environmental Remediation		Asset Impairment and Accelerated Depreciation			Other			Total
	(In millions)
Reserve at September 30, 2018	$	1		$	4	$	—		$	—		$	5
Charges (gain)		7			—		1			1			9
Costs charged against assets / (liabilities)		—			1		(1	)		—			—
Cash paid		(1	)		—		—			(1	)		(2	)
Reserve at December 31, 2018		7			5		—			—			12
Charges (gain)		1			—		—			1			2
Costs charged against assets / (liabilities)		—			—		—			—			—
Cash paid		(1	)		—		—			(1	)		(2	)
Reserve at March 31, 2019		7			5		—			—			12
Charges (gain)		2			—		1			1			4
Costs charged against assets / (liabilities)		—			—		(1	)					(1	)
Cash paid		(2	)		—		—			(1	)		(3	)
Reserve at June 30, 2019	$	7		$	5	$	—		$	—		$	12

Derivatives (Tables)

9 Months Ended

Jun. 30, 2019

Derivative Instruments And Hedging Activities Disclosure [Abstract]

Details of Derivatives Held to Manage Foreign Currency Risk

The following table provides details of the derivatives held as of June 30, 2019 and September 30, 2018 to manage foreign currency risk.

		Notional Amount
Description	Borrowing	June 30, 2019	September 30, 2018	Hedge Designation
Cross-Currency Swaps	3.4% Notes	USD 250 million swapped to EUR 223 million	USD 250 million swapped to EUR 223 million	Net investment
Forward Foreign Currency Contracts ⁽¹⁾	N/A	USD 28 million	USD 18 million	No designation

⁽¹⁾	Cabot’s forward foreign exchange contracts are denominated in the Canadian dollar, Indonesian rupiah and Czech koruna.

Summary Impact of Cross-currency Swaps to AOCI and Consolidated Statements of Operations

The following tables summarize the impact of the cross-currency swaps to AOCI and the Consolidated Statements of Operations:

	Three Months Ended June 30
	2019			2018		2019			2018			2019		2018
Description	Gain/(Loss) Recognized in AOCI					(Gain)/Loss Reclassified from AOCI into Interest Expense in the Consolidated Statements of Operations						(Gain)/Loss Recognized in Interest Expense in the Consolidated Statements of Operations (Amount Excluded from Effectiveness Testing)
	(In millions)
Cross-currency swaps	$	(1	)	$	10	$	(1	)	$	(2	)	$	—	$	—

	Nine Months Ended June 30
	2019		2018			2019			2018			2019		2018
Description	Gain/(Loss) Recognized in AOCI					(Gain)/Loss Reclassified from AOCI into Interest Expense in the Consolidated Statements of Operations						(Gain)/Loss Recognized in Interest Expense in the Consolidated Statements of Operations (Amount Excluded from Effectiveness Testing)
	(In millions)
Cross-currency swaps	$	12	$	(3	)	$	(4	)	$	(4	)	$	1	$	1

Tabular disclosure of pertinent information about a derivative or group of derivatives on a disaggregated basis, such as for individual instruments, or small groups of similar instruments. May include a combination of the type of instrument, risks being hedged, notional amount, hedge designation, related hedged item, inception date, maturity date, or other relevant item.

+ References

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Reference 10: http://fasb.org/us-gaap/role/ref/legacyRef
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+ Details

Financial Information by Segment (Tables)

9 Months Ended

Jun. 30, 2019

Schedule of Performance Segment

The net sales from each of these businesses for the three and nine months ended June 30, 2019 and 2018 were as follows:

	Three Months Ended June 30				Nine Months Ended June 30
	2019		2018		2019		2018
	(In millions)
Performance Additives	$	172	$	188	$	518	$	524
Formulated Solutions		79		86		218		247
Performance Chemicals	$	251	$	274	$	736	$	771

Financial Information by Reportable Segment

Financial information by reportable segment is as follows:

	Reinforcement Materials		Performance Chemicals		Purification Solutions			Specialty Fluids			Segment Total		Unallocated and Other⁽¹⁾			Consolidated Total
	(In millions)
Three Months Ended June 30, 2019
Revenues from external customers⁽²⁾	$	461	$	251	$	73		$	13		$	798	$	47		$	845
Income (loss) from continuing operations before income taxes⁽³⁾	$	72	$	37	$	1		$	2		$	112	$	(43	)	$	69

Three Months Ended June 30, 2018
Revenues from external customers⁽²⁾	$	466	$	274	$	70		$	12		$	822	$	32		$	854
Income (loss) from continuing operations before income taxes⁽³⁾	$	74	$	56	$	(6	)	$	3		$	127	$	(32	)	$	95

Nine Months Ended June 30, 2019
Revenues from external customers⁽²⁾	$	1,363	$	736	$	210		$	56		$	2,365	$	145		$	2,510
Income (loss) from continuing operations before income taxes⁽³⁾	$	195	$	111	$	(1	)	$	24		$	329	$	(141	)	$	188

Nine Months Ended June 30, 2018
Revenues from external customers⁽²⁾	$	1,307	$	771	$	206		$	24		$	2,308	$	84		$	2,392
Income (loss) from continuing operations before income taxes⁽³⁾	$	215	$	160	$	(6	)	$	(2	)	$	367	$	(351	)	$	16

⁽¹⁾	Unallocated and Other includes certain items and eliminations necessary to reflect management’s reporting of operating segment results. These items are reflective of the segment reporting presented to the CODM.

⁽²⁾

	Three Months Ended June 30						Nine Months Ended June 30
	2019			2018			2019			2018
	(In millions)
Royalties, the impact of unearned revenue, the removal of 100% of the sales of an equity method affiliate and discounting charges for certain notes receivable	$	(3	)	$	(2	)	$	(9	)	$	(10	)
Shipping and handling fees		32			34			96			94
By-product sales^(a)		18			—			58			—
Total	$	47		$	32		$	145		$	84

^(a)

⁽³⁾

	Three Months Ended June 30						Nine Months Ended June 30
	2019			2018			2019			2018
	(In millions)
Interest expense	$	(14	)	$	(14	)	$	(43	)	$	(41	)
Certain items^(a)
Specialty Fluids loss on sale and asset impairment charge (Note D)		(8	)		—			(28	)		—
Equity affiliate investment impairment charge (Note O)		—			—			(11	)		—
Purification Solutions goodwill and long-lived assets impairment charge (Note F)		—			—			—			(254	)
Inventory reserve adjustment (Note F)		—			—			—			(13	)
Global restructuring activities (Note L)		(4	)		(1	)		(15	)		7
Legal and environmental matters and reserves		—			—			(1	)		(6	)
Gains (losses) on sale of investments		—			—			—			10
Acquisition and integration-related charges		(1	)		—			(5	)		(1	)
Other		(1	)		(2	)		(1	)		(3	)
Total certain items, pre-tax		(14	)		(3	)		(61	)		(260	)
Unallocated corporate costs^(b)		(14	)		(15	)		(39	)		(45	)
General unallocated income (expense)^(c)		—			—			3			(3	)
Less: Equity in earnings of affiliated companies, net of tax^(d)		1			—			1			2
Total	$	(43	)	$	(32	)	$	(141	)	$	(351	)

^(a)

^(b)

Unallocated corporate costs are costs that are not controlled by the segments and primarily benefit corporate interests.

^(c)

^(d)

Revenues from External Customers by Geographic Region

	Three Months Ended June 30, 2019
	Reinforcement Materials		Performance Chemicals		Purification Solutions		Specialty Fluids		Consolidated Total
	(In millions)
Americas	$	179	$	69	$	33	$	2	$	283
Asia Pacific		191		93		10		—		294
Europe, Middle East and Africa		91		89		30		11		221
Segment revenues from external customers		461		251		73		13		798
Unallocated and other										47
Net sales and other operating revenues									$	845

	Nine Months Ended June 30, 2019
	Reinforcement Materials		Performance Chemicals		Purification Solutions		Specialty Fluids		Consolidated Total
	(In millions)
Americas	$	515	$	222	$	93	$	6	$	836
Asia Pacific		573		253		26		1		853
Europe, Middle East and Africa		275		261		91		49		676
Segment revenues from external customers		1,363		736		210		56		2,365
Unallocated and other										145
Net sales and other operating revenues									$	2,510

Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended		3 Months Ended		9 Months Ended
	Nov. 30, 2018	Nov. 30, 2013	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Significant Accounting Policies [Line Items]
Net sales and other operating revenues			$ 845.0	$ 854.0	$ 2,510.0	$ 2,392.0
Cost of sales			675.0	$ 657.0	1,996.0	$ 1,831.0
Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Significant Accounting Policies [Line Items]
Net sales and other operating revenues			(18.0)		(58.0)
Cost of sales			$ (18.0)		$ (58.0)
NHUMO [Member]
Significant Accounting Policies [Line Items]
Preferred stock repurchase amount	$ 25.0	$ 25.0
Dividend percentage of preferred stock		6.00%
Preferred stock dividend	$ 1.4
Minimum [Member]
Significant Accounting Policies [Line Items]
Payment terms					0 days
Minimum [Member] \| Buildings [Member]
Significant Accounting Policies [Line Items]
Useful life of property, plant and equipment					20 years
Minimum [Member] \| Machinery and Equipment [Member]
Significant Accounting Policies [Line Items]
Useful life of property, plant and equipment					10 years
Minimum [Member] \| Other Fixed Assets [Member]
Significant Accounting Policies [Line Items]
Useful life of property, plant and equipment					3 years
Maximum [Member]
Significant Accounting Policies [Line Items]
Payment terms					90 days
Maximum [Member] \| Buildings [Member]
Significant Accounting Policies [Line Items]
Useful life of property, plant and equipment					25 years
Maximum [Member] \| Machinery and Equipment [Member]
Significant Accounting Policies [Line Items]
Useful life of property, plant and equipment					25 years
Maximum [Member] \| Other Fixed Assets [Member]
Significant Accounting Policies [Line Items]
Useful life of property, plant and equipment					25 years

Significant Accounting Policies - Schedule of Change Retrospectively to Prior Periods Consolidated Statements of Operations (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Significant Accounting Policies [Line Items]
Cost of sales	$ 675	$ 657	$ 1,996	$ 1,831
Gross profit	170	197	514	561
Selling and administrative expenses	65	74	208	223
Research and technical expenses	16	17	47	48
Income (loss) from operations	$ 81	106	231	36
Other income (expense)		1	$ (6)	13
Change in Method of Accounting for Inventories from Last-in, First-out Method to First-in, First-out Method [Member]
Significant Accounting Policies [Line Items]
Cost of sales		657		1,831
Gross profit		197		561
Selling and administrative expenses		74		223
Research and technical expenses		17		48
Income (loss) from operations		106		36
Other income (expense)		1		13
Change in Method of Accounting for Inventories from Last-in, First-out Method to First-in, First-out Method [Member] \| As Originally Reported [Member]
Significant Accounting Policies [Line Items]
Cost of sales		654		1,824
Gross profit		200		568
Selling and administrative expenses		74		221
Research and technical expenses		17		48
Income (loss) from operations		109		45
Other income (expense)		(2)		4
Change in Method of Accounting for Inventories from Last-in, First-out Method to First-in, First-out Method [Member] \| Effect of Change [Member]
Significant Accounting Policies [Line Items]
Cost of sales		3		7
Gross profit		(3)		(7)
Selling and administrative expenses				2
Income (loss) from operations		(3)		(9)
Other income (expense)		$ 3		$ 9

Acquisitions - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended			3 Months Ended	9 Months Ended
	Jun. 30, 2019	Sep. 30, 2018	Nov. 30, 2017	Jun. 30, 2019	Jun. 30, 2019
Tech Blend [Member]
Business Acquisition [Line Items]
Acquisition date			Nov. 30, 2017
Business acquisition, purchase price			$ 65
NSCC Carbon (Jiangsu) Co. Ltd [Member]
Business Acquisition [Line Items]
Acquisition date		Sep. 30, 2018
Expected Production commencement year		2021
Transition related costs				$ 1	$ 4
NSCC Carbon (Jiangsu) Co. Ltd [Member] \| Accounts Payable and Accrued Liabilities [Member]
Business Acquisition [Line Items]
Business acquisition, purchase price		$ 8
Other Acquisition [Member] \| Intangible Assets [Member]
Business Acquisition [Line Items]
Business acquisition, purchase price	$ 3

Sale of the Specialty Fluids Business - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2019
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total proceeds from sale of Specialty Fluids business				$ 130
Proceeds held in escrow				5
Specialty Fluids loss on sale and asset impairment (Note D)		$ 8		$ 28
Sinomine (Hong Kong) Rare Metals Resources Co. Limited [Member] \| Specialty Fluids [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total proceeds from sale of Specialty Fluids business	$ 133
Amount of cash transferred	2
Proceeds held in escrow	5
Pre-tax loss on sale of Specialty Fluids business		$ 8
Specialty Fluids loss on sale and asset impairment (Note D)			$ 20
Additional cash royalties for lithium products	$ 5
Royalty payable term	10 years

Employee Benefit Plans - Net Periodic Defined Benefit Pension and Other Postretirement Benefit Costs (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Pension Benefits [Member] \| U.S. Defined Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1	$ 1	$ 1	$ 1
Interest cost	1	1	4	3
Expected return on plan assets	(2)	(3)	(7)	(8)
Net periodic benefit (credit) cost		(1)	(2)	(4)
Pension Benefits [Member] \| Foreign Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	2	2	5	6
Interest cost	1	2	4	6
Expected return on plan assets	(2)	(4)	(8)	(11)
Amortization of prior service cost (credit)	2		2
Amortization of actuarial loss	1	1	2	2
Settlement and curtailment gain			(6)
Net periodic benefit (credit) cost	4	1	(1)	3
Postretirement Benefits [Member] \| U.S. Defined Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost		1	1	1
Amortization of prior service cost (credit)		$ (1)	(1)	(2)
Amortization of actuarial loss	(1)		(1)
Net periodic benefit (credit) cost	$ (1)		$ (1)	$ (1)

Employee Benefit Plans - Additional Information (Detail) $ in Millions	3 Months Ended
	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($) Plan
Defined Benefit Plan Disclosure [Line Items]
Number of foreign defined benefit plans \| Plan		1
Reduction in net pension obligations		$ (30)
Pre-tax gain from net pension obligations		6
Settlement credit		5
Plan assets remain to be transferred.	$ 1
Additional charge on defined benefit plan actuarial gain (loss)		$ 3
Pay out period for additional charge payment		5 years
Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Curtailment credit		$ 1
U.K. Plan [Member] \| Other Income (Expense) [Member]
Defined Benefit Plan Disclosure [Line Items]
Charge on effect of accued benefit	$ 2

Purification Solutions Goodwill and Long-Lived Assets Impairment Charges - Impairment Charges and Associated Tax Benefit (Detail) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Goodwill And Intangible Assets [Line Items]
Purification Solutions goodwill impairment charge					$ 92
Purification Solutions long-lived assets impairment charge					162
Benefit for income taxes				$ (20)	$ 131
Impairment charges, after tax	$ 8			$ 28
Purification Solutions [Member]
Goodwill And Intangible Assets [Line Items]
Purification Solutions goodwill impairment charge		$ 92	$ 92
Purification Solutions long-lived assets impairment charge			162
Benefit for income taxes			(30)
Impairment charges, after tax			$ 224

Purification Solutions Goodwill and Long-Lived Assets Impairment Charges - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2018
Goodwill And Intangible Assets [Line Items]
Goodwill impairment charge			$ 92
Long-lived assets impairment charge			$ 162
Assets impairment charges income tax benefit	$ 30
Reducing of impairment charges and restructuring costs	1
Purification Solutions [Member]
Goodwill And Intangible Assets [Line Items]
Goodwill impairment charge	$ 92	$ 92
Lower of cost or market reserve inventory		13
Long-lived assets impairment charge		162
Impairment charges of definite lived Intangible assets		64
Purification Solutions [Member] \| Property, Plant and Equipment [Member]
Goodwill And Intangible Assets [Line Items]
Long-lived assets impairment charge		$ 98

Goodwill and Intangible Assets - Schedule of Goodwill Balances (Detail) $ in Millions	9 Months Ended
	Jun. 30, 2019 USD ($)
Goodwill And Intangible Assets [Line Items]
Beginning balance	$ 93
Foreign currency impact	(1)
Ending balance	92
Reinforcement Materials [Member]
Goodwill And Intangible Assets [Line Items]
Beginning balance	52
Foreign currency impact	(1)
Ending balance	51
Performance Chemicals [Member]
Goodwill And Intangible Assets [Line Items]
Beginning balance	41
Ending balance	$ 41

Goodwill and Intangible Assets - Schedule of Intangible Assets (Detail) - USD ($) $ in Millions	Jun. 30, 2019	Sep. 30, 2018
Finite Lived Intangible Assets [Line Items]
Gross Carrying Value, finite lives	$ 117	$ 111
Accumulated Amortization	(17)	(13)
Net Intangible Assets, finite lives	100	98
Developed Technologies [Member]
Finite Lived Intangible Assets [Line Items]
Gross Carrying Value, finite lives	51	52
Accumulated Amortization	(4)	(2)
Net Intangible Assets, finite lives	47	50
Trademarks [Member]
Finite Lived Intangible Assets [Line Items]
Gross Carrying Value, finite lives	8	8
Net Intangible Assets, finite lives	8	8
Customer Relationships [Member]
Finite Lived Intangible Assets [Line Items]
Gross Carrying Value, finite lives	58	51
Accumulated Amortization	(13)	(11)
Net Intangible Assets, finite lives	$ 45	$ 40

Goodwill and Intangible Assets - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Goodwill And Intangible Assets [Line Items]
Amortization expense estimated for year one	$ 6		$ 6
Amortization expense estimated for year two	6		6
Amortization expense estimated for year three	6		6
Amortization expense estimated for year four	6		6
Amortization expense estimated for year five	6		6
Cost of Sales, Selling and Administrative Expenses and Research and Technical Expenses [Member]
Goodwill And Intangible Assets [Line Items]
Amortization of intangible assets	$ 1	$ 1	$ 4	$ 6
Minimum [Member]
Goodwill And Intangible Assets [Line Items]
Useful life of intangible assets			12 years
Maximum [Member]
Goodwill And Intangible Assets [Line Items]
Useful life of intangible assets			25 years
Weighted Average [Member]
Goodwill And Intangible Assets [Line Items]
Useful life of intangible assets			19 years

Accumulated Other Comprehensive Income (Loss) - Changes in Each Component of AOCI, Net of Tax (Detail) - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	$ 1,232	$ 1,276	$ 1,279	$ 1,343	$ 1,279	$ 1,625
Ending Balance	1,226	1,232	1,276	1,289	1,226	1,289
Currency Translation Adjustment [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	(286)	(291)	(266)		(266)
Other comprehensive income (loss) before reclassifications	1	8	(24)
Amounts reclassified from AOCI		(1)	(1)
Less: Other comprehensive income (loss) attributable to noncontrolling interests	(2)	2
Ending Balance	(283)	(286)	(291)		(283)
Unrealized Gains on Investments [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	1	1	1		1
Ending Balance	1	1	1		1
Pension and Other Postretirement Benefit Liability Adjustments [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	(30)	(30)	(52)		(52)
Other comprehensive income (loss) before reclassifications	(1)	1	30
Amounts reclassified from AOCI	2	(1)	(8)
Ending Balance	(29)	(30)	(30)		(29)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	(315)	(320)	(317)	(209)	(317)	(259)
Other comprehensive income (loss) before reclassifications		9	6
Amounts reclassified from AOCI	2	(2)	(9)	(1)	(9)	(3)
Less: Other comprehensive income (loss) attributable to noncontrolling interests	(2)	2
Ending Balance	$ (311)	$ (315)	$ (320)	$ (313)	$ (311)	$ (313)

Accumulated Other Comprehensive Income (Loss) - Amounts Reclassified Out of AOCI (Detail) - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
(Gains) Losses Reclassified to Interest Expense [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	$ (1)			$ (2)	$ (4)	$ (4)
(Gains) Losses Excluded from Effectiveness Testing and Amortized to Interest Expense [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax					1
Amortization of Actuarial Losses [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax				1	1	2
Amortization of Prior Service Cost (Credit) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	2			(1)	1	(2)
Settlement and Curtailment Gain [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax					(6)
Accumulated Other Comprehensive Income (Loss) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	1			(2)	(7)	(4)
Tax impact	1			1	(2)	1
Total after tax	$ 2	$ (2)	$ (9)	$ (1)	$ (9)	$ (3)

Commitments and Contingencies - Schedule of Components of Purchase Commitments (Detail) $ in Millions	Jun. 30, 2019 USD ($)
Long-term Purchase Commitment [Line Items]
Payments Due by Remainder of Fiscal Year 2019	$ 128
Payments Due by Fiscal Year 2020	320
Payments Due by Fiscal Year 2021	199
Payments Due by Fiscal Year 2022	188
Payments Due by Fiscal Year 2023	157
Payments Due Thereafter	2,094
Payments Due, Total	3,086
Reinforcement Materials [Member]
Long-term Purchase Commitment [Line Items]
Payments Due by Remainder of Fiscal Year 2019	100
Payments Due by Fiscal Year 2020	251
Payments Due by Fiscal Year 2021	139
Payments Due by Fiscal Year 2022	131
Payments Due by Fiscal Year 2023	123
Payments Due Thereafter	1,584
Payments Due, Total	2,328
Performance Chemicals [Member]
Long-term Purchase Commitment [Line Items]
Payments Due by Remainder of Fiscal Year 2019	27
Payments Due by Fiscal Year 2020	63
Payments Due by Fiscal Year 2021	59
Payments Due by Fiscal Year 2022	57
Payments Due by Fiscal Year 2023	34
Payments Due Thereafter	510
Payments Due, Total	750
Purification Solutions [Member]
Long-term Purchase Commitment [Line Items]
Payments Due by Remainder of Fiscal Year 2019	1
Payments Due by Fiscal Year 2020	6
Payments Due by Fiscal Year 2021	1
Payments Due, Total	$ 8

Commitments and Contingencies - Additional Information (Detail) $ in Millions	9 Months Ended
	Jun. 30, 2019 USD ($) claimants	Jun. 30, 2018 USD ($)	Sep. 30, 2018 USD ($) claimants
Respirator Liabilities [Member]
Loss Contingencies [Line Items]
Number of claimants \| claimants	36,000		35,000
Cash payments for respirator reserves	$ 4	$ 1
Respirator reserve	21		$ 25
Environmental Matters [Member]
Loss Contingencies [Line Items]
Reserved for environmental matters	14		$ 15
Cash payments for environmental reserves	$ 1	$ 2

Income Tax - Effective Tax Rate (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Tax Disclosure [Abstract]
(Provision) benefit for income taxes	$ (30)	$ 4	$ (43)	$ (194)
Effective tax rate	43.00%	(3.00%)	23.00%	1202.00%

Income Tax - Additional Information (Detail) - USD ($)		3 Months Ended				9 Months Ended
Income Tax - Additional Information (Detail) - USD ($)	Jan. 01, 2018	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2019	Jun. 30, 2018
Income Taxes [Line Items]
Net discrete tax (benefits) charges related to tax settlements		$ 13,000,000		$ (23,000,000)		$ (11,000,000)	$ 139,000,000
Net tax expense from impact of the Act		17,000,000		(4,000,000)		$ 0	185,000,000
Income tax rate	21.00%				35.00%
Percentage of dividend received deduction for foreign dividends						100.00%
Benefit for income taxes for deemed repatriation			$ 17,000,000
Additional tax expense due to reversal		17,000,000
Uncertain tax positions from expirations of statute of limitations		$ 1,000,000		$ 0		$ 9,000,000	$ 2,000,000
Earliest Tax Year [Member] \| United States Internal Revenue Service (IRS) [Member]
Income Taxes [Line Items]
Tax years remain subject to examination						2014
Latest Tax Year [Member] \| United States Internal Revenue Service (IRS) [Member]
Income Taxes [Line Items]
Tax years remain subject to examination						2017
State Tax Authorities [Member] \| Earliest Tax Year [Member]
Income Taxes [Line Items]
Tax years remain subject to examination						2005
State Tax Authorities [Member] \| Latest Tax Year [Member]
Income Taxes [Line Items]
Tax years remain subject to examination						2017
Non-U.S. Jurisdictions [Member] \| Earliest Tax Year [Member]
Income Taxes [Line Items]
Tax years remain subject to examination						2002
Non-U.S. Jurisdictions [Member] \| Latest Tax Year [Member]
Income Taxes [Line Items]
Tax years remain subject to examination						2017

Earnings Per Share - Components of Basic and Diluted Earnings (Loss) Per Common Share (EPS) (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Basic EPS:
Net income (loss) attributable to Cabot Corporation	$ 32	$ 88	$ 124	$ (207)
Earnings (loss) allocated to common shareholders (numerator)	$ 32	$ 88	$ 124	$ (207)
Weighted average common shares and participating securities outstanding	59.0	62.5	59.9	62.5
Less: Participating securities	0.8	0.7	0.8	0.7
Adjusted weighted average common shares (denominator)	58.2	61.8	59.1	61.8
Earnings (loss) per common share - basic:	$ 0.55	$ 1.41	$ 2.08	$ (3.36)
Diluted EPS:
Earnings (loss) allocated to common shareholders	$ 32	$ 88	$ 124	$ (207)
Plus: Earnings (loss) allocated to participating securities		1	1	1
Less: Adjusted earnings allocated to participating securities		1	1	1
Earnings (loss) allocated to common shareholders (numerator)	$ 32	$ 88	$ 124	$ (207)
Adjusted weighted average common shares outstanding	58.2	61.8	59.1	61.8
Common shares issuable	0.2	0.5	0.1
Adjusted weighted average common shares (denominator)	58.4	62.3	59.2	61.8
Earnings (loss) per common share - diluted:	$ 0.55	$ 1.40	$ 2.08	$ (3.36)

Earnings Per Share - Calculation of Undistributed Earnings (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Earnings Per Share [Abstract]
Net income (loss) attributable to Cabot Corporation	$ 32	$ 88	$ 124	$ (207)
Less: Dividends declared on common stock	20	21	60	60
Undistributed earnings (loss)	12	67	64	(267)
Undistributed earnings (loss) allocated to common shareholders	$ 12	$ 67	$ 64	$ (267)

Net Investment Hedge [Member] | Cross Currency Swaps [Member]

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Earnings Per Share - Components of Basic and Diluted Earnings (Loss) Per Common Share (EPS) (Parenthetical) (Detail) - shares	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Earnings Per Share [Abstract]
Antidilutive securities excluded from computation of earnings per share	983,081	260,630	945,246	799,163

Restructuring - Recorded Restructuring Activities (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Restructuring Cost And Reserve [Line Items]
Restructuring reserve, period expense	$ 4	$ 1	$ 15	$ (7)
Cost of Sales [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring reserve, period expense	2	$ 1	7	(8)
Selling and Administrative Expenses [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring reserve, period expense	$ 2		$ 8	$ 1

Restructuring - Restructuring Activities and Related Reserves (Detail) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2019
Restructuring Cost And Reserve [Line Items]
Reserve balance	$ 12	$ 12	$ 5	$ 5
Charges (gain)	4	2	9
Costs charged against assets / (liabilities)	(1)
Cash paid	(3)	(2)	(2)
Reserve balance	12	12	12	12
Severance and Employee Benefits [Member]
Restructuring Cost And Reserve [Line Items]
Reserve balance	7	7	1	1
Charges (gain)	2	1	7
Cash paid	(2)	(1)	(1)
Reserve balance	7	7	7	7
Environmental Remediation [Member]
Restructuring Cost And Reserve [Line Items]
Reserve balance	5	5	4	4
Costs charged against assets / (liabilities)			1
Reserve balance	5	5	5	$ 5
Asset Impairment and Accelerated Depreciation [Member]
Restructuring Cost And Reserve [Line Items]
Charges (gain)	1		1
Costs charged against assets / (liabilities)	(1)		(1)
Other [Member]
Restructuring Cost And Reserve [Line Items]
Charges (gain)	1	1	1
Cash paid	$ (1)	$ (1)	$ (1)

Restructuring - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended					9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Restructuring Cost And Reserve [Line Items]
Restructuring Charges		$ 4	$ 2	$ 9
Proceeds from sale of land							$ 13
Purification Solutions Transformation Plan [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring Charges						$ 9
Severance costs						8
Professional fees						1
Restructuring charges expected remainder of fiscal year and thereafter		1				1
Restructuring cash payments of fiscal year		2				4
Accrued severance charges		5				$ 5
Purification Solutions Transformation Plan [Member] \| Maximum [Member]
Restructuring Cost And Reserve [Line Items]
Severance costs		$ 1
Purification Solutions Transformation Plan [Member] \| Scenario, Forecast [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring cash payments through the reminder of fiscal 2019 and thereafter	$ 6
Operational Restructuring [Member] \| Merak, Indonesia [Member] \| Cost of Sales [Member]
Restructuring Cost And Reserve [Line Items]
Proceeds from sale of land					$ 13
Net benefit to earnings					$ 11

Financial Instruments and Fair Value Measurements - Additional Information (Detail)	1 Months Ended	9 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	May 31, 2019 EUR (€)	Sep. 30, 2018 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, liabilities, Level 2 to Level 1 transfers, amount	$ 0	$ 0	$ 0
Fair value, assets, Level 2 to Level 1 transfers, amount	0	0	0
Fair value, liabilities, Level 1 to Level 2 transfers, amount	0	0	0
Fair value, assets, Level 1 to Level 2 transfers, amount	0	0	0
Fair value, assets, transfers into Level 3, amount		0	0
Fair value, assets, transfers out of Level 3, amount		0	0
Fair value, liabilities, transfers into Level 3, amount		0	0
Fair value, liabilities, transfers out of Level 3, amount		0	$ 0
Fair value of long-term debt	1,077,000,000	$ 1,077,000,000			$ 742,000,000
4% Notes mature on 2029 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt issuance cost	3,000,000
Registered unsecured notes issued	$ 300,000,000
Debt Instrument, interest rate	4.00%	4.00%
Debt maturity date	Jul. 01, 2029
Proceeds from issuance of unsecured notes	$ 296,000,000
Debt instrument, discount	1,000,000	$ 1,000,000
Revolving Credit Agreement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Revolving credit facility, committed amount \| €				€ 300,000,000
Credit agreement, available borrowing	294,000,000	$ 294,000,000
Debt instrument, maturity date		May 22, 2024
Debt issuance cost		$ 1,000,000
Unsecured Revolving Credit Agreement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Revolving credit facility, committed amount	1,000,000,000	1,000,000,000
Fixed Rate Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value of long-term debt	1,018,000,000	1,018,000,000			747,000,000
Significant Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets in the Consolidated Balance Sheets	11,000,000	11,000,000			11,000,000
Significant Observable Inputs (Level 2) [Member] \| Foreign Currency Risks [Member] \| Prepaid Expenses and Other Current Assets and Other Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets (liabilities), at fair Value, net	$ (8,000,000)	$ (8,000,000)			$ (18,000,000)

Derivatives - Details of Derivatives Held to Manage Foreign Currency Risk (Detail)

Jun. 30, 2019

USD ($)

Jun. 30, 2019

EUR (€)

Sep. 30, 2018

USD ($)

Sep. 30, 2018

EUR (€)

Net Investment Hedge [Member] | 3.4% Notes [Member] | Cross Currency Swaps [Member]

$ 250,000,000

Net Investment Hedge [Member] | 3.4% Notes [Member] | Fixed Rate Debt [Member] | Cross Currency Swaps [Member]

$ 250,000,000

€ 223,000,000

$ 250,000,000

€ 223,000,000

Interest rate on borrowing

3.40%

Not Designated as Hedging Instrument [Member] | Forward Foreign Currency Contracts [Member]

^[1]

$ 28,000,000

$ 18,000,000

[1]	Cabot’s forward foreign exchange contracts are denominated in the Canadian dollar, Indonesian rupiah and Czech koruna.

Derivatives - Additional Information (Detail) - Cross Currency Swaps [Member] - USD ($)	9 Months Ended
	Jun. 30, 2019	Sep. 30, 2018
Prepaid Expenses And Other Asset Current And Other Liabilities [Member]
Derivative [Line Items]
Fair value of swaps, net liability	$ 8,000,000	$ 18,000,000
Net Investment Hedge [Member]
Derivative [Line Items]
Notional Amount	250,000,000
Cumulative gain (loss) related to swaps	$ (5,000,000)	$ (14,000,000)
Net Investment Hedge [Member] \| Fixed Rate Debt [Member]
Derivative [Line Items]
Derivative Instruments Frequency of Cash Settlements	semi-annually

Derivatives - Summary Impact of Cross-currency Swaps to AOCI and Consolidated Statements of Operations (Detail) - Cross Currency Swaps [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Derivative Instruments Gain Loss [Line Items]
Gain/(Loss) Recognized in AOCI	$ (1)	$ 10	$ 12	$ (3)
(Gain)/Loss Reclassified from AOCI into Interest Expense in the Consolidated Statements of Operations	$ (1)	$ (2)	(4)	(4)
(Gain)/Loss Recognized in Interest Expense in the Consolidated Statements of Operations (Amount Excluded from Effectiveness Testing)			$ 1	$ 1

Hyperinflationary Economies - Additional Information (Detail)	3 Months Ended	9 Months Ended	12 Months Ended
	Jun. 30, 2019 USD ($) $ / $	Jun. 30, 2019 USD ($) $ / $	Sep. 30, 2018
Argentina [Member]
Schedule Of Equity Method Investments [Line Items]
Percentage of net revenue	2.00%	3.00%
Foreign currency exchange rate, translation \| $ / $	42.7	42.7
Argentina [Member] \| Other Income (Expense) [Member]
Schedule Of Equity Method Investments [Line Items]
Foreign currency exchange gains or (losses)		$ (1,000,000)
Argentina [Member] \| Maximum [Member]
Schedule Of Equity Method Investments [Line Items]
Percentage of assets and liabilities current		2.00%	2.00%
Argentina [Member] \| Maximum [Member] \| Other Income (Expense) [Member]
Schedule Of Equity Method Investments [Line Items]
Foreign currency exchange gains or (losses)	$ 1,000,000
Venezuela [Member] \| Carbon Black Affiliate [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	49.00%	49.00%
Venezuela [Member] \| Other Income (Expense) [Member] \| Carbon Black Affiliate [Member]
Schedule Of Equity Method Investments [Line Items]
Impairment charge	$ 11,000,000
Carbon Black Operating Entity [Member] \| Argentina [Member]
Schedule Of Equity Method Investments [Line Items]
Ownership percentage	100.00%	100.00%

Financial Information by Segment - Additional Information (Detail) - Segment		3 Months Ended	9 Months Ended
	Jun. 28, 2019	Jun. 30, 2019	Jun. 30, 2019
Segment Reporting Information [Line Items]
Number of business reportable segments	3	4
Performance Chemicals [Member]
Segment Reporting Information [Line Items]
Number of business reportable segments			1

Financial Information by Segment - Schedule of Performance Segment (Detail) - Performance Chemicals [Member] - Operating Segments [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Net sales	$ 251	$ 274	$ 736	$ 771
Performance Additives [Member]
Segment Reporting Information [Line Items]
Net sales	172	188	518	524
Formulated Solutions [Member]
Segment Reporting Information [Line Items]
Net sales	$ 79	$ 86	$ 218	$ 247

Financial Information by Segment - Financial Information by Reportable Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Revenues from external customers	$ 845	$ 854	$ 2,510	$ 2,392
Income (loss) from continuing operations before income taxes	69	95	188	16
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	798	822	2,365	2,308
Income (loss) from continuing operations before income taxes	112	127	329	367
Unallocated and Other [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	47	32	145	84
Income (loss) from continuing operations before income taxes	(43)	(32)	(141)	(351)
Reinforcement Materials [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	461	466	1,363	1,307
Income (loss) from continuing operations before income taxes	72	74	195	215
Performance Chemicals [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	251	274	736	771
Income (loss) from continuing operations before income taxes	37	56	111	160
Purification Solutions [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	73	70	210	206
Income (loss) from continuing operations before income taxes	1	(6)	(1)	(6)
Specialty Fluids [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	13	12	56	24
Income (loss) from continuing operations before income taxes	$ 2	$ 3	$ 24	$ (2)

Financial Information by Segment - Financial Information by Reportable Segment (Parenthetical) (Detail)	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting [Abstract]
Percentage of sale of an equity method affiliate	100.00%	100.00%	100.00%	100.00%

Financial Information by Segment - Sales of Equity Method Affiliate and Discounting Charges for Certain Notes Receivable and By-product Revenue (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Revenues from external customers	$ 845	$ 854	$ 2,510	$ 2,392
Unallocated and Other [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	47	32	145	84
Unallocated and Other [Member] \| Royalties, the Impact of Unearned Revenue, the Removal of 100% of the Sales of an Equity Method Affiliate and Discounting Charges for Certain Notes Receivable [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	(3)	(2)	(9)	(10)
Unallocated and Other [Member] \| Shipping and Handling Costs [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	32	$ 34	96	$ 94
Unallocated and Other [Member] \| By-product Sales [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	$ 18		$ 58

Financial Information by Segment - Sales of Equity Method Affiliate and Discounting Charges for Certain Notes Receivable and By-product Revenue (Parenthetical) (Detail)	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting [Abstract]
Percentage of sale of an equity method affiliate	100.00%	100.00%	100.00%	100.00%

Financial Information by Segment - Schedule of Income (Loss) from Continuing Operations before Income Taxes and Equity in Earnings of Affiliated Companies for Unallocated and Other (Detail) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Interest expense	$ (14)	$ (14)		$ (43)	$ (41)
Goodwill and long-lived assets impairment charge	(8)			(28)
Global restructuring activities	(4)	(1)		(15)	7
Gains (losses) on sale of investments					10
Specialty Fluids [Member]
Segment Reporting Information [Line Items]
Loss on sale and asset impairment charge	(8)			(28)
Purification Solutions [Member]
Segment Reporting Information [Line Items]
Goodwill and long-lived assets impairment charge			$ (224)
Unallocated and Other [Member]
Segment Reporting Information [Line Items]
Interest expense	(14)	(14)		(43)	(41)
Unallocated corporate costs	(14)	(15)		(39)	(45)
General unallocated income (expense)				3	(3)
Less: Equity in earnings of affiliated companies, net of tax	1			1	2
Income from continuing operations before income taxes and equity in earnings of affiliated companies	(43)	(32)		(141)	(351)
Certain Item [Member]
Segment Reporting Information [Line Items]
Inventory reserve adjustment					(13)
Global restructuring activities	(4)	(1)		(15)	7
Legal and environmental matters and reserves				(1)	(6)
Gains (losses) on sale of investments					10
Acquisition and integration-related charges	(1)			(5)	(1)
Other	(1)	(2)		(1)	(3)
Total certain items, pre-tax	$ (14)	$ (3)		(61)	(260)
Certain Item [Member] \| Equity Affiliate Investment [Member]
Segment Reporting Information [Line Items]
Investment impairment charge				$ (11)
Certain Item [Member] \| Purification Solutions [Member]
Segment Reporting Information [Line Items]
Goodwill and long-lived assets impairment charge					$ (254)

Financial Information by Segment - Revenue from External Customers by Geographic Region (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Net sales and other operating revenues	$ 845	$ 854	$ 2,510	$ 2,392
Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	798	822	2,365	2,308
Unallocated and Other [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	47	32	145	84
Americas [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	283		836
Asia Pacific [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	294		853
Europe, Middle East and Africa [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	221		676
Reinforcement Materials [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	461	466	1,363	1,307
Reinforcement Materials [Member] \| Americas [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	179		515
Reinforcement Materials [Member] \| Asia Pacific [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	191		573
Reinforcement Materials [Member] \| Europe, Middle East and Africa [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	91		275
Performance Chemicals [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	251	274	736	771
Performance Chemicals [Member] \| Americas [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	69		222
Performance Chemicals [Member] \| Asia Pacific [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	93		253
Performance Chemicals [Member] \| Europe, Middle East and Africa [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	89		261
Purification Solutions [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	73	70	210	206
Purification Solutions [Member] \| Americas [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	33		93
Purification Solutions [Member] \| Asia Pacific [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	10		26
Purification Solutions [Member] \| Europe, Middle East and Africa [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	30		91
Specialty Fluids [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	13	$ 12	56	$ 24
Specialty Fluids [Member] \| Americas [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	2		6
Specialty Fluids [Member] \| Asia Pacific [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues			1
Specialty Fluids [Member] \| Europe, Middle East and Africa [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	$ 11		$ 49

Document and Entity Information - shares	9 Months Ended
Document and Entity Information - shares	Jun. 30, 2019	Aug. 06, 2019
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q3
Trading Symbol	CBT
Entity Registrant Name	CABOT CORP
Entity Central Index Key	0000016040
Entity Current Reporting Status	Yes
Current Fiscal Year End Date	--09-30
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		57,782,998
Entity Shell Company	false
Entity Small Business	false
Entity Emerging Growth Company	false
Entity File Number	1-5667
Entity Tax Identification Number	042271897
Entity Address, Address Line One	Two Seaport Lane
Entity Address, City or Town	Boston
Entity Address, State or Province	Massachusetts
Entity Address, Postal Zip Code	02210-2019
City Area Code	617
Local Phone Number	345-0100

Consolidated Statements of Operations (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Statement [Abstract]
Net sales and other operating revenues	$ 845	$ 854	$ 2,510	$ 2,392
Cost of sales	675	657	1,996	1,831
Gross profit	170	197	514	561
Selling and administrative expenses	65	74	208	223
Research and technical expenses	16	17	47	48
Specialty Fluids loss on sale and asset impairment (Note D)	8		28
Purification Solutions long-lived assets impairment (Note F)				162
Purification Solutions goodwill impairment (Note F)				92
Income (loss) from operations	81	106	231	36
Interest and dividend income	2	2	6	8
Interest expense	(14)	(14)	(43)	(41)
Other income (expense)		1	(6)	13
Income (loss) from continuing operations before income taxes and equity in earnings of affiliated companies	69	95	188	16
(Provision) benefit for income taxes	(30)	4	(43)	(194)
Equity in earnings of affiliated companies, net of tax	1		1	2
Net income (loss)	40	99	146	(176)
Net income (loss) attributable to noncontrolling interests, net of tax	8	11	22	31
Net income (loss) attributable to Cabot Corporation	$ 32	$ 88	$ 124	$ (207)
Weighted-average common shares outstanding:
Basic	58.2	61.8	59.1	61.8
Diluted	58.4	62.3	59.2	61.8
Earnings per common share:
Basic	$ 0.55	$ 1.41	$ 2.08	$ (3.36)
Diluted	0.55	1.40	2.08	(3.36)
Dividends per common share	$ 0.35	$ 0.33	$ 1.01	$ 0.96

Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ 40	$ 99	$ 146	$ (176)
Other comprehensive income (loss), net of tax
Foreign currency translation adjustment, net of tax (provision) benefit of $—, $(2), $(3) and $2	1	(109)	(15)	(50)
Unrealized holding gains (losses) arising during the period, net of tax (provision) benefit of $—, $—, $— and $—		(1)		(1)
Derivatives: net investment hedges
(Gains) losses reclassified to interest expense, net of tax provision (benefit) of $—, $1, $1 and $1	(1)	(1)	(3)	(3)
(Gains) losses excluded from effectiveness testing and amortized to interest expense, net of tax provision (benefit) of $1, $—, $— and $—	1		1
Pension and other postretirement benefit liability adjustments
Pension and other postretirement benefit liability adjustments arising during the period, net of tax	(1)		(23)
Other comprehensive income (loss)	2	(111)	6	(54)
Comprehensive income (loss)	42	(12)	152	(230)
Net income (loss) attributable to noncontrolling interests, net of tax	8	11	22	31
Foreign currency translation adjustment attributable to noncontrolling interests, net of tax	(2)	(7)
Comprehensive income (loss) attributable to noncontrolling interests, net of tax	6	4	22	31
Comprehensive income (loss) attributable to Cabot Corporation	$ 36	$ (16)	$ 130	$ (261)

Consolidated Statements of Comprehensive Income (Parenthetical) (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Statement Of Income And Comprehensive Income [Abstract]
Foreign currency translation adjustment, tax provision (benefit)		$ (2)	$ (3)	$ 2
(Gains) losses reclassified to interest expense, tax provision (benefit)		$ 1	$ 1	$ 1
(Gains) losses excluded from effectiveness testing and amortized to interest expense, tax provision (benefit)	$ 1

Consolidated Balance Sheets (Unaudited) - USD ($) $ in Millions	Jun. 30, 2019	Sep. 30, 2018
Current assets:
Cash and cash equivalents	$ 147	$ 175
Accounts and notes receivable, net of reserve for doubtful accounts of $4 and $7	611	637
Inventories:
Raw materials	125	129
Work in process		3
Finished goods	340	329
Other	53	50
Total inventories	518	511
Prepaid expenses and other current assets	59	63
Total current assets	1,335	1,386
Property, plant and equipment, net	1,336	1,296
Goodwill	92	93
Equity affiliates	39	52
Intangible assets, net	100	98
Assets held for rent		118
Deferred income taxes	146	134
Other assets	71	67
Total assets	3,119	3,244
Current liabilities:
Short-term borrowings	84	249
Accounts payable and accrued liabilities	553	613
Income taxes payable	3	29
Current portion of long-term debt	5	35
Redeemable preferred stock		26
Total current liabilities	645	952
Long-term debt	1,017	719
Deferred income taxes	40	42
Other liabilities	191	252
Commitments and contingencies (Note I)
Preferred stock:
Authorized: 2,000,000 shares of $1 par value Issued and Outstanding: None and none
Common stock:
Authorized: 200,000,000 shares of $1 par value Issued: 57,952,863 and 60,566,375 shares Outstanding: 57,758,660 and 60,366,569 shares	58	61
Less cost of 194,203 and 199,806 shares of common treasury stock	(6)	(7)
Retained earnings	1,351	1,417
Accumulated other comprehensive income (loss)	(311)	(317)
Total Cabot Corporation stockholders' equity	1,092	1,154
Noncontrolling interests	134	125
Total stockholders' equity	1,226	1,279
Total liabilities and stockholders' equity	$ 3,119	$ 3,244

Consolidated Balance Sheets (Parenthetical) (Unaudited) - USD ($) $ in Millions	Jun. 30, 2019	Sep. 30, 2018
Statement Of Financial Position [Abstract]
Accounts and notes receivable, reserve for doubtful accounts	$ 4	$ 7
Preferred stock, authorized shares	2,000,000	2,000,000
Preferred stock, par value	$ 1	$ 1
Preferred stock, Issued shares	0	0
Preferred stock, Outstanding shares	0	0
Common stock, authorized shares	200,000,000	200,000,000
Common stock, par value	$ 1	$ 1
Common stock, issued shares	57,952,863	60,566,375
Common stock, outstanding shares	57,758,660	60,366,569
Common treasury stock, shares	194,203	199,806

Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Millions	9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash Flows from Operating Activities:
Net income (loss)	$ 146	$ (176)
Adjustments to reconcile net income (loss) to cash provided by operating activities:
Depreciation and amortization	110	117
Specialty Fluids loss on sale and asset impairment	28
Impairment of investment in equity affiliate	11
Long-lived assets impairment charge		162
Goodwill impairment charge		92
Deferred tax provision (benefit)	(20)	131
Gain on sale of investments		(10)
Employee benefit plan settlement	6
Gain on sale of land		(11)
Equity in net income of affiliated companies	(1)	(2)
Non-cash compensation	11	16
Other non-cash (income) expense	(2)	14
Changes in assets and liabilities:
Accounts and notes receivable	6	(151)
Inventories	(14)	(77)
Prepaid expenses and other current assets	(1)	(6)
Accounts payable and accrued liabilities	(65)	40
Income taxes payable	(24)	2
Other liabilities	(27)	(6)
Cash dividends received from equity affiliates	2	8
Cash provided (used) by operating activities	166	143
Cash Flows from Investing Activities:
Additions to property, plant and equipment	(155)	(167)
Proceeds from sale of business, net of cash held in escrow of $5 and $—	130
Cash paid for acquisition of business, net of cash acquired of $— and $1	(3)	(64)
Proceeds from sale of investments		11
Proceeds from sale of land		13
Change in assets held for rent	(8)	(5)
Other	3	1
Cash provided (used) by investing activities	(33)	(211)
Cash Flows from Financing Activities:
Borrowings under financing arrangements	29
Repayments under financing arrangements	(18)	(1)
Increase in short-term borrowings, net		2
Proceeds from (repayments of) issuance of commercial paper, net	(176)	303
Proceeds from long-term debt, net of issuance costs	344
Repayments of long-term debt	(75)	(251)
Repayments of redeemable preferred stock	(25)
Purchases of common stock	(144)	(59)
Proceeds from sales of common stock	2	18
Cash dividends paid to noncontrolling interests	(23)	(21)
Cash dividends paid to common stockholders	(60)	(60)
Cash provided (used) by financing activities	(146)	(69)
Effects of exchange rate changes on cash	(15)	(12)
Increase (decrease) in cash and cash equivalents	(28)	(149)
Cash and cash equivalents at beginning of period	175	280
Cash and cash equivalents at end of period	$ 147	$ 131

Consolidated Statements of Cash Flows (Parenthetical) (Unaudited) $ in Millions	9 Months Ended
	Jun. 30, 2018 USD ($)
Statement Of Cash Flows [Abstract]
Cash acquired in acquisition of business	$ 1

Consolidated Statements of Changes in Stockholders' Equity (Unaudited) - USD ($) shares in Thousands, $ in Millions	Total	Common Stock, Net of Treasury Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Total Cabot Corporation Stockholders' Equity [Member]	Noncontrolling Interests [Member]
Beginning Balance at Sep. 30, 2017	$ 1,625	$ 56		$ 1,707	$ (259)	$ 1,504	$ 121
Beginning Balance, Shares at Sep. 30, 2017		61,884
Net income (loss)	(112)			(122)		(122)	10
Total other comprehensive income (loss)	(1)				(4)	(4)	3
Cash dividends declared to noncontrolling interests	(10)						(10)
Cash dividends paid, Common stock, per share	(20)			(20)		(20)
Issuance of stock under equity compensation plans, Shares		178
Amortization of share-based compensation	6		$ 6			6
Purchase and retirement of common stock	(16)		(6)	(10)		(16)
Purchase and retirement of common stock, Shares		(265)
Ending Balance at Dec. 31, 2017	1,472	$ 56		1,555	(263)	1,348	124
Ending Balance, Shares at Dec. 31, 2017		61,797
Beginning Balance at Sep. 30, 2017	1,625	$ 56		1,707	(259)	1,504	121
Beginning Balance, Shares at Sep. 30, 2017		61,884
Net income (loss)	(176)
Total other comprehensive income (loss)	(54)
Ending Balance at Jun. 30, 2018	1,289	$ 55		1,417	(313)	1,159	130
Ending Balance, Shares at Jun. 30, 2018		61,559
Beginning Balance at Dec. 31, 2017	1,472	$ 56		1,555	(263)	1,348	124
Beginning Balance, Shares at Dec. 31, 2017		61,797
Net income (loss)	(163)			(173)		(173)	10
Total other comprehensive income (loss)	58				54	54	4
Cash dividends declared to noncontrolling interests	(11)						(11)
Cash dividends paid, Common stock, per share	(19)			(19)		(19)
Issuance of stock under equity compensation plans	1		1			1
Issuance of stock under equity compensation plans, Shares		30
Amortization of share-based compensation	6		6			6
Purchase and retirement of common stock	(1)	$ (1)	(7)	7		(1)
Purchase and retirement of common stock, Shares		(8)
Ending Balance at Mar. 31, 2018	1,343	$ 55		1,370	(209)	1,216	127
Ending Balance, Shares at Mar. 31, 2018		61,819
Net income (loss)	99			88		88	11
Total other comprehensive income (loss)	(111)				(104)	(104)	(7)
Cash dividends declared to noncontrolling interests	(1)						(1)
Cash dividends paid, Common stock, per share	(21)			(21)		(21)
Issuance of stock under equity compensation plans	16		16			16
Issuance of stock under equity compensation plans, Shares		421
Amortization of share-based compensation	4		4			4
Purchase and retirement of common stock	(40)		(20)	(20)		(40)
Purchase and retirement of common stock, Shares		(681)
Ending Balance at Jun. 30, 2018	1,289	$ 55		1,417	(313)	1,159	130
Ending Balance, Shares at Jun. 30, 2018		61,559
Beginning Balance at Sep. 30, 2018	1,279	$ 54		1,417	(317)	1,154	125
Beginning Balance, Shares at Sep. 30, 2018		60,367
Net income (loss)	77			69		69	8
Total other comprehensive income (loss)	(3)				(3)	(3)
Cash dividends paid, Common stock, per share	(20)			(20)		(20)
Issuance of stock under equity compensation plans, Shares		344
Amortization of share-based compensation	5		5			5
Purchase and retirement of common stock	(62)	$ (1)	(5)	(56)		(62)
Purchase and retirement of common stock, Shares		(1,201)
Ending Balance at Dec. 31, 2018	1,276	$ 53		1,410	(320)	1,143	133
Ending Balance, Shares at Dec. 31, 2018		59,510
Beginning Balance at Sep. 30, 2018	1,279	$ 54		1,417	(317)	1,154	125
Beginning Balance, Shares at Sep. 30, 2018		60,367
Net income (loss)	146
Total other comprehensive income (loss)	6
Ending Balance at Jun. 30, 2019	1,226	$ 52		1,351	(311)	1,092	134
Ending Balance, Shares at Jun. 30, 2019		57,759
Beginning Balance at Dec. 31, 2018	1,276	$ 53		1,410	(320)	1,143	133
Beginning Balance, Shares at Dec. 31, 2018		59,510
Net income (loss)	29			23		23	6
Total other comprehensive income (loss)	7				5	5	2
Cash dividends declared to noncontrolling interests	(13)						(13)
Cash dividends paid, Common stock, per share	(20)			(20)		(20)
Issuance of stock under equity compensation plans	1	$ 1				1
Issuance of stock under equity compensation plans, Shares		41
Amortization of share-based compensation	2		2			2
Purchase and retirement of common stock	(50)	$ (2)	(2)	(46)		(50)
Purchase and retirement of common stock, Shares		(1,101)
Ending Balance at Mar. 31, 2019	1,232	$ 52		1,367	(315)	1,104	128
Ending Balance, Shares at Mar. 31, 2019		58,450
Net income (loss)	40			32		32	8
Total other comprehensive income (loss)	2				4	4	(2)
Cash dividends paid, Common stock, per share	(20)			(20)		(20)
Issuance of stock under equity compensation plans, Shares		56
Amortization of share-based compensation	4		4			4
Purchase and retirement of common stock	(32)		$ (4)	(28)		(32)
Purchase and retirement of common stock, Shares		(747)
Ending Balance at Jun. 30, 2019	$ 1,226	$ 52		$ 1,351	$ (311)	$ 1,092	$ 134
Ending Balance, Shares at Jun. 30, 2019		57,759

Consolidated Statements of Changes in Stockholders' Equity (Unaudited) (Parenthetical) - $ / shares	3 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Statement Of Stockholders Equity [Abstract]
Common stock, cash dividends paid, per share	$ 0.35	$ 0.33	$ 0.33	$ 0.33	$ 0.315	$ 0.315

Basis of Presentation

9 Months Ended

Jun. 30, 2019

Basis of Presentation

A. Basis of Presentation

Significant Accounting Policies

9 Months Ended

Jun. 30, 2019

Significant Accounting Policies

B. Significant Accounting Policies

Revenue Recognition

period incurred and included in Cost of sales within the Consolidated Statement of Operations. Taxes collected on sales to customers are excluded from the transaction price.

The Company does not have contract assets or liabilities that are material.

Intangible Assets and Goodwill Impairment

Long-lived Assets Impairment

Property, Plant and Equipment

Income Tax in Interim Periods

Inventory Valuation

Pensions and Other Postretirement Benefits

Redeemable Preferred Stock

Accumulated Other Comprehensive Income (Loss)

Recently Adopted Accounting Standards

Consolidated Statements of Operations	Three Months Ended June 30								Nine Months Ended June 30
	2018								2018
	As Originally Reported			Effect of Change			As Adjusted		As Originally Reported		Effect of Change			As Adjusted
	(In millions, except per share amounts)
Cost of sales	$	654		$	3		$	657	$	1,824	$	7		$	1,831
Gross profit	$	200		$	(3	)	$	197	$	568	$	(7	)	$	561
Selling and administrative expenses	$	74		$	—		$	74	$	221	$	2		$	223
Research and technical expenses	$	17		$	—		$	17	$	48	$	—		$	48
Income (loss) from operations	$	109		$	(3	)	$	106	$	45	$	(9	)	$	36
Other income (expense)	$	(2	)	$	3		$	1	$	4	$	9		$	13

Recent Accounting Pronouncements

Acquisitions

9 Months Ended

Jun. 30, 2019

Business Combinations [Abstract]

Acquisitions

C. Acquisitions

Tech Blend

NSCC Carbon (Jiangsu) Co. Ltd

Other Acquisition

Sale of the Specialty Fluids Business

9 Months Ended

Jun. 30, 2019

Discontinued Operations And Disposal Groups [Abstract]

Sale of the Specialty Fluids Business

Note D. Sale of the Specialty Fluids Business

As part of the sale, the Company may receive additional cash royalties of up to $5 million for lithium products, payable over a ten-year period.

Employee Benefit Plans

9 Months Ended

Jun. 30, 2019

Compensation And Retirement Disclosure [Abstract]

Employee Benefit Plans

E. Employee Benefit Plans

Net periodic defined benefit pension and other postretirement benefit costs include the following:

	Three Months Ended June 30
	2019						2018						2019					2018
	Pension Benefits												Postretirement Benefits
	U.S.			Foreign			U.S.			Foreign			U.S.			Foreign		U.S.			Foreign
	(In millions)
Service cost	$	1		$	2		$	1		$	2		$	—		$	—	$	—		$	—
Interest cost		1			1			1			2			—			—		1			—
Expected return on plan assets		(2	)		(2	)		(3	)		(4	)		—			—		—			—
Amortization of prior service cost (credit)		—			2			—			—			—			—		(1	)		—
Amortization of actuarial loss		—			1			—			1			(1	)		—		—			—
Settlement and curtailment gain		—			—			—			—			—			—		—			—
Net periodic benefit (credit) cost	$	—		$	4		$	(1	)	$	1		$	(1	)	$	—	$	—		$	—

	Nine Months Ended June 30
	2019						2018						2019					2018
	Pension Benefits												Postretirement Benefits
	U.S.			Foreign			U.S.			Foreign			U.S.			Foreign		U.S.			Foreign
	(In millions)
Service cost	$	1		$	5		$	1		$	6		$	—		$	—	$	—		$	—
Interest cost		4			4			3			6			1			—		1			—
Expected return on plan assets		(7	)		(8	)		(8	)		(11	)		—			—		—			—
Amortization of prior service cost (credit)		—			2			—			—			(1	)		—		(2	)		—
Amortization of actuarial loss		—			2			—			2			(1	)		—		—			—
Settlement and curtailment gain		—			(6	)		—			—			—			—		—			—
Net periodic benefit (credit) cost	$	(2	)	$	(1	)	$	(4	)	$	3		$	(1	)	$	—	$	(1	)	$	—

Curtailments and Settlements of Employee Benefit Plans

Purification Solutions Goodwill and Long-Lived Assets Impairment Charges

9 Months Ended

Jun. 30, 2019

Goodwill And Intangible Assets Disclosure [Abstract]

Purification Solutions Goodwill and Long-Lived Assets Impairment Charges

F. Purification Solutions Goodwill and Long-Lived Assets Impairment Charges

	Three Months Ended March 31, 2018
	(In millions)
Purification Solutions goodwill impairment charge	$	92
Purification Solutions long-lived assets impairment charge		162
Benefit for income taxes		(30	)
Impairment charges, after tax	$	224

Cabot will continue to monitor for events or changes in business circumstances that may indicate that the remaining carrying value of the asset group may not be recoverable.

Goodwill and Intangible Assets

9 Months Ended

Jun. 30, 2019

Goodwill And Intangible Assets Disclosure [Abstract]

Goodwill and Intangible Assets

G. Goodwill and Intangible Assets

The carrying amount of goodwill attributable to each reportable segment with goodwill balances and the changes in those balances during the nine month period ended June 30, 2019 are as follows:

	Reinforcement Materials			Performance Chemicals		Total
	(In millions)
Balance at September 30, 2018	$	52		$	41	$	93
Foreign currency impact		(1	)		—		(1	)
Balance at June 30, 2019	$	51		$	41	$	92

The following table provides information regarding the Company’s intangible assets:

	June 30, 2019							September 30, 2018
	Gross Carrying Value		Accumulated Amortization			Net Intangible Assets		Gross Carrying Value		Accumulated Amortization			Net Intangible Assets
	(In millions)
Intangible assets with finite lives
Developed technologies	$	51	$	(4	)	$	47	$	52	$	(2	)	$	50
Trademarks		8		—			8		8		—			8
Customer relationships		58		(13	)		45		51		(11	)		40
Total intangible assets	$	117	$	(17	)	$	100	$	111	$	(13	)	$	98

Accumulated Other Comprehensive Income (Loss)

9 Months Ended

Jun. 30, 2019

Stockholders Equity Note [Abstract]

Commitments And Contingencies Disclosure [Abstract]

H. Accumulated Other Comprehensive Income (Loss)

Comprehensive income combines net income (loss) and other comprehensive income items, which are reported as components of stockholders’ equity in the accompanying Consolidated Balance Sheets.

Changes in each component of AOCI, net of tax, were as follows:

	Currency Translation Adjustment			Unrealized Gains on Investments		Pension and Other Postretirement Benefit Liability Adjustments			Total
	(In millions)
Balance at September 30, 2018, attributable to Cabot Corporation	$	(266	)	$	1	$	(52	)	$	(317	)
Other comprehensive income (loss) before reclassifications		(24	)		—		30			6
Amounts reclassified from AOCI		(1	)		—		(8	)		(9	)
Less: Other comprehensive income (loss) attributable to noncontrolling interests		—			—		—			—
Balance at December 31, 2018, attributable to Cabot Corporation	$	(291	)	$	1	$	(30	)	$	(320	)
Other comprehensive income (loss) before reclassifications		8			—		1			9
Amounts reclassified from AOCI		(1	)		—		(1	)		(2	)
Less: Other comprehensive income (loss) attributable to noncontrolling interests		2			—		—			2
Balance at March 31, 2019, attributable to Cabot Corporation		(286	)		1		(30	)		(315	)
Other comprehensive income (loss) before reclassifications		1			—		(1	)		—
Amounts reclassified from AOCI		—			—		2			2
Less: Other comprehensive income (loss) attributable to noncontrolling interests		(2	)		—		—			(2	)
Balance at June 30, 2019, attributable to Cabot Corporation	$	(283	)	$	1	$	(29	)	$	(311	)

The amounts reclassified out of AOCI and into the Consolidated Statements of Operations in the three and nine months ended June 30, 2019 and 2018 were as follows:

	Affected Line Item in the Consolidated	Three Months Ended June 30						Nine Months Ended June 30
	Statements of Operations	2019			2018			2019			2018
		(In millions)
Derivatives: net investment hedges
(Gains) losses reclassified to interest expense	Interest expense	$	(1	)	$	(2	)	$	(4	)	$	(4	)
(Gains) losses excluded from effectiveness testing and amortized to interest expense	Interest expense	$	—		$	—		$	1		$	—
Pension and other postretirement benefit liability adjustment
Amortization of actuarial losses	Net Periodic Benefit Cost - see Note E for details		—			1			1			2
Amortization of prior service cost (credit)	Net Periodic Benefit Cost - see Note E for details		2			(1	)		1			(2	)
Settlement and curtailment gain	Net Periodic Benefit Cost - see Note E for details		—			—			(6	)		—
Total before tax			1			(2	)		(7	)		(4	)
Tax impact	Provision (benefit) for income taxes		1			1			(2	)		1
Total after tax		$	2		$	(1	)	$	(9	)	$	(3	)

Commitments and Contingencies

9 Months Ended

Jun. 30, 2019

Commitments and Contingencies

I. Commitments and Contingencies

Purchase Commitments

	Payments Due by Fiscal Year
	Remainder of Fiscal 2019		2020		2021		2022		2023		Thereafter		Total
	(In millions)
Reinforcement Materials	$	100	$	251	$	139	$	131	$	123	$	1,584	$	2,328
Performance Chemicals		27		63		59		57		34		510		750
Purification Solutions		1		6		1		—		—		—		8
Total	$	128	$	320	$	199	$	188	$	157	$	2,094	$	3,086

Guarantee Agreements

Contingencies

Cabot is a defendant, or potentially responsible party, in various lawsuits and environmental proceedings wherein substantial amounts are claimed or at issue.

Environmental Matters

Respirator Liabilities

Other Matters

Income Tax

9 Months Ended

Jun. 30, 2019

Income Tax

J. Income Tax

Effective Tax Rate

	Three Months Ended June 30						Nine Months Ended June 30
	2019			2018			2019			2018
	(Dollars in millions)
(Provision) benefit for income taxes	$	(30	)	$	4		$	(43	)	$	(194	)
Effective tax rate		43	%		(3	)%		23	%		1202	%

Tax Reform

Uncertainties

Earnings Per Share

9 Months Ended

Jun. 30, 2019

Restructuring And Related Activities [Abstract]

Earnings Per Share

K. Earnings Per Share

The following tables summarize the components of the basic and diluted earnings (loss) per common share (“EPS”) computations:

	Three Months Ended June 30				Nine Months Ended June 30
	2019		2018		2019		2018
	(In millions, except per share amounts)
Basic EPS:
Net income (loss) attributable to Cabot Corporation	$	32	$	88	$	124	$	(207	)
Less: Undistributed earnings allocated to participating securities⁽¹⁾		—		—		—		—
Earnings (loss) allocated to common shareholders (numerator)	$	32	$	88	$	124	$	(207	)

Weighted average common shares and participating securities outstanding		59.0		62.5		59.9		62.5
Less: Participating securities⁽¹⁾		0.8		0.7		0.8		0.7
Adjusted weighted average common shares (denominator)		58.2		61.8		59.1		61.8

Earnings (loss) per common share - basic:	$	0.55	$	1.41	$	2.08	$	(3.36	)

Diluted EPS:
Earnings (loss) allocated to common shareholders	$	32	$	88	$	124	$	(207	)
Plus: Earnings (loss) allocated to participating securities		—		1		1		1
Less: Adjusted earnings allocated to participating securities⁽²⁾		—		1		1		1
Earnings (loss) allocated to common shareholders (numerator)	$	32	$	88	$	124	$	(207	)

Adjusted weighted average common shares outstanding		58.2		61.8		59.1		61.8
Effect of dilutive securities:
Common shares issuable⁽³⁾		0.2		0.5		0.1		—
Adjusted weighted average common shares (denominator)		58.4		62.3		59.2		61.8

Earnings (loss) per common share - diluted:	$	0.55	$	1.40	$	2.08	$	(3.36	)

⁽¹⁾	^{Participating securities consist of shares underlying outstanding and achieved performance-based restricted stock units issued during and after fiscal 2017 and all unvested time-based restricted stock units.}

	Three Months Ended June 30				Nine Months Ended June 30
	2019		2018		2019		2018
	(In millions)
Calculation of undistributed earnings (loss):
Net income (loss) attributable to Cabot Corporation	$	32	$	88	$	124	$	(207	)
Less: Dividends declared on common stock		20		21		60		60
Undistributed earnings (loss)	$	12	$	67	$	64	$	(267	)

Allocation of undistributed earnings (loss):
Undistributed earnings (loss) allocated to common shareholders	$	12	$	67	$	64	$	(267	)
Undistributed earnings (loss)	$	12	$	67	$	64	$	(267	)

⁽²⁾	Undistributed earnings are adjusted for the assumed distribution of dividends to the dilutive securities, which are described in (3) below, and then reallocated to participating securities.

⁽³⁾

Restructuring

9 Months Ended

Jun. 30, 2019

Restructuring

L. Restructuring

Cabot’s restructuring activities were recorded in the Consolidated Statements of Operations in the three and nine months ended June 30, 2019 and 2018 as follows:

	Three Months Ended June 30				Nine Months Ended June 30
	2019		2018		2019		2018
	(In millions)
Cost of sales	$	2	$	1	$	7	$	(8	)
Selling and administrative expenses		2		—		8		1
Total	$	4	$	1	$	15	$	(7	)

Details of all restructuring activities and the related reserves during the three and nine months ended June 30, 2019 were as follows:

	Severance and Employee Benefits			Environmental Remediation		Asset Impairment and Accelerated Depreciation			Other			Total
	(In millions)
Reserve at September 30, 2018	$	1		$	4	$	—		$	—		$	5
Charges (gain)		7			—		1			1			9
Costs charged against assets / (liabilities)		—			1		(1	)		—			—
Cash paid		(1	)		—		—			(1	)		(2	)
Reserve at December 31, 2018		7			5		—			—			12
Charges (gain)		1			—		—			1			2
Costs charged against assets / (liabilities)		—			—		—			—			—
Cash paid		(1	)		—		—			(1	)		(2	)
Reserve at March 31, 2019		7			5		—			—			12
Charges (gain)		2			—		1			1			4
Costs charged against assets / (liabilities)		—			—		(1	)					(1	)
Cash paid		(2	)		—		—			(1	)		(3	)
Reserve at June 30, 2019	$	7		$	5	$	—		$	—		$	12

Purification Solutions Transformation Plan

2016 Plan

Financial Instruments and Fair Value Measurements

9 Months Ended

Jun. 30, 2019

Fair Value Disclosures [Abstract]

Financial Instruments and Fair Value Measurements

M. Financial Instruments and Fair Value Measurements

Level 1	—	Quoted market prices in active markets for identical assets or liabilities

Level 2	—	Significant other observable inputs (e.g., quoted prices for similar items in active markets, quoted prices for identical or similar items in markets that are not active, inputs other than quoted prices that are observable such as interest rate and yield curves, and market-corroborated inputs)

Level 3	—	Significant unobservable inputs

Derivatives

9 Months Ended

Jun. 30, 2019

Derivative Instruments And Hedging Activities Disclosure [Abstract]

Derivatives

N. Derivatives

Foreign Currency Risk Management

The following table provides details of the derivatives held as of June 30, 2019 and September 30, 2018 to manage foreign currency risk.

		Notional Amount
Description	Borrowing	June 30, 2019	September 30, 2018	Hedge Designation
Cross-Currency Swaps	3.4% Notes	USD 250 million swapped to EUR 223 million	USD 250 million swapped to EUR 223 million	Net investment
Forward Foreign Currency Contracts ⁽¹⁾	N/A	USD 28 million	USD 18 million	No designation

⁽¹⁾	Cabot’s forward foreign exchange contracts are denominated in the Canadian dollar, Indonesian rupiah and Czech koruna.

Accounting for Derivative Instruments and Hedging Activities

Fair Value Hedge

Cash Flow Hedge

Net Investment Hedge

The following tables summarize the impact of the cross-currency swaps to AOCI and the Consolidated Statements of Operations:

	Three Months Ended June 30
	2019			2018		2019			2018			2019		2018
Description	Gain/(Loss) Recognized in AOCI					(Gain)/Loss Reclassified from AOCI into Interest Expense in the Consolidated Statements of Operations						(Gain)/Loss Recognized in Interest Expense in the Consolidated Statements of Operations (Amount Excluded from Effectiveness Testing)
	(In millions)
Cross-currency swaps	$	(1	)	$	10	$	(1	)	$	(2	)	$	—	$	—

	Nine Months Ended June 30
	2019		2018			2019			2018			2019		2018
Description	Gain/(Loss) Recognized in AOCI					(Gain)/Loss Reclassified from AOCI into Interest Expense in the Consolidated Statements of Operations						(Gain)/Loss Recognized in Interest Expense in the Consolidated Statements of Operations (Amount Excluded from Effectiveness Testing)
	(In millions)
Cross-currency swaps	$	12	$	(3	)	$	(4	)	$	(4	)	$	1	$	1

Other Derivative Instruments

Hyperinflationary Economies

9 Months Ended

Jun. 30, 2019

Equity Method Investments And Joint Ventures [Abstract]

Hyperinflationary Economies

O. Hyperinflationary Economies

Argentina

The Company will continue to monitor the developments in Argentina and their potential impact on the operating entity’s operations or carrying value.

Venezuela

Financial Information by Segment

9 Months Ended

Jun. 30, 2019

Financial Information by Segment

P. Financial Information by Segment

The Reinforcement Materials segment consists of the rubber blacks and elastomer composites product lines.

	Three Months Ended June 30				Nine Months Ended June 30
	2019		2018		2019		2018
	(In millions)
Performance Additives	$	172	$	188	$	518	$	524
Formulated Solutions		79		86		218		247
Performance Chemicals	$	251	$	274	$	736	$	771

Financial information by reportable segment is as follows:

	Reinforcement Materials		Performance Chemicals		Purification Solutions			Specialty Fluids			Segment Total		Unallocated and Other⁽¹⁾			Consolidated Total
	(In millions)
Three Months Ended June 30, 2019
Revenues from external customers⁽²⁾	$	461	$	251	$	73		$	13		$	798	$	47		$	845
Income (loss) from continuing operations before income taxes⁽³⁾	$	72	$	37	$	1		$	2		$	112	$	(43	)	$	69

Three Months Ended June 30, 2018
Revenues from external customers⁽²⁾	$	466	$	274	$	70		$	12		$	822	$	32		$	854
Income (loss) from continuing operations before income taxes⁽³⁾	$	74	$	56	$	(6	)	$	3		$	127	$	(32	)	$	95

Nine Months Ended June 30, 2019
Revenues from external customers⁽²⁾	$	1,363	$	736	$	210		$	56		$	2,365	$	145		$	2,510
Income (loss) from continuing operations before income taxes⁽³⁾	$	195	$	111	$	(1	)	$	24		$	329	$	(141	)	$	188

Nine Months Ended June 30, 2018
Revenues from external customers⁽²⁾	$	1,307	$	771	$	206		$	24		$	2,308	$	84		$	2,392
Income (loss) from continuing operations before income taxes⁽³⁾	$	215	$	160	$	(6	)	$	(2	)	$	367	$	(351	)	$	16

⁽¹⁾	Unallocated and Other includes certain items and eliminations necessary to reflect management’s reporting of operating segment results. These items are reflective of the segment reporting presented to the CODM.

⁽²⁾

	Three Months Ended June 30						Nine Months Ended June 30
	2019			2018			2019			2018
	(In millions)
Royalties, the impact of unearned revenue, the removal of 100% of the sales of an equity method affiliate and discounting charges for certain notes receivable	$	(3	)	$	(2	)	$	(9	)	$	(10	)
Shipping and handling fees		32			34			96			94
By-product sales^(a)		18			—			58			—
Total	$	47		$	32		$	145		$	84

^(a)

⁽³⁾

	Three Months Ended June 30						Nine Months Ended June 30
	2019			2018			2019			2018
	(In millions)
Interest expense	$	(14	)	$	(14	)	$	(43	)	$	(41	)
Certain items^(a)
Specialty Fluids loss on sale and asset impairment charge (Note D)		(8	)		—			(28	)		—
Equity affiliate investment impairment charge (Note O)		—			—			(11	)		—
Purification Solutions goodwill and long-lived assets impairment charge (Note F)		—			—			—			(254	)
Inventory reserve adjustment (Note F)		—			—			—			(13	)
Global restructuring activities (Note L)		(4	)		(1	)		(15	)		7
Legal and environmental matters and reserves		—			—			(1	)		(6	)
Gains (losses) on sale of investments		—			—			—			10
Acquisition and integration-related charges		(1	)		—			(5	)		(1	)
Other		(1	)		(2	)		(1	)		(3	)
Total certain items, pre-tax		(14	)		(3	)		(61	)		(260	)
Unallocated corporate costs^(b)		(14	)		(15	)		(39	)		(45	)
General unallocated income (expense)^(c)		—			—			3			(3	)
Less: Equity in earnings of affiliated companies, net of tax^(d)		1			—			1			2
Total	$	(43	)	$	(32	)	$	(141	)	$	(351	)

^(a)

^(b)

Unallocated corporate costs are costs that are not controlled by the segments and primarily benefit corporate interests.

^(c)

^(d)

	Three Months Ended June 30, 2019
	Reinforcement Materials		Performance Chemicals		Purification Solutions		Specialty Fluids		Consolidated Total
	(In millions)
Americas	$	179	$	69	$	33	$	2	$	283
Asia Pacific		191		93		10		—		294
Europe, Middle East and Africa		91		89		30		11		221
Segment revenues from external customers		461		251		73		13		798
Unallocated and other										47
Net sales and other operating revenues									$	845

	Nine Months Ended June 30, 2019
	Reinforcement Materials		Performance Chemicals		Purification Solutions		Specialty Fluids		Consolidated Total
	(In millions)
Americas	$	515	$	222	$	93	$	6	$	836
Asia Pacific		573		253		26		1		853
Europe, Middle East and Africa		275		261		91		49		676
Segment revenues from external customers		1,363		736		210		56		2,365
Unallocated and other										145
Net sales and other operating revenues									$	2,510

Significant Accounting Policies (Policies)

9 Months Ended

Jun. 30, 2019

Intangible Assets and Goodwill Impairment

Revenue Recognition

period incurred and included in Cost of sales within the Consolidated Statement of Operations. Taxes collected on sales to customers are excluded from the transaction price.

The Company does not have contract assets or liabilities that are material.

Long-Lived Assets Impairment

Long-lived Assets Impairment

Property, Plant and Equipment

Income Tax in Interim Periods

Inventory Valuation

Pensions and Other Postretirement Benefits

Redeemable Preferred Stock

Recently Adopted Accounting Standards

Consolidated Statements of Operations	Three Months Ended June 30								Nine Months Ended June 30
	2018								2018
	As Originally Reported			Effect of Change			As Adjusted		As Originally Reported		Effect of Change			As Adjusted
	(In millions, except per share amounts)
Cost of sales	$	654		$	3		$	657	$	1,824	$	7		$	1,831
Gross profit	$	200		$	(3	)	$	197	$	568	$	(7	)	$	561
Selling and administrative expenses	$	74		$	—		$	74	$	221	$	2		$	223
Research and technical expenses	$	17		$	—		$	17	$	48	$	—		$	48
Income (loss) from operations	$	109		$	(3	)	$	106	$	45	$	(9	)	$	36
Other income (expense)	$	(2	)	$	3		$	1	$	4	$	9		$	13

Recent Accounting Pronouncements

Significant Accounting Policies (Tables)

9 Months Ended

Jun. 30, 2019

Schedule of Change Retrospectively to Prior Periods Financial Statement Line Items

Consolidated Statements of Operations	Three Months Ended June 30								Nine Months Ended June 30
	2018								2018
	As Originally Reported			Effect of Change			As Adjusted		As Originally Reported		Effect of Change			As Adjusted
	(In millions, except per share amounts)
Cost of sales	$	654		$	3		$	657	$	1,824	$	7		$	1,831
Gross profit	$	200		$	(3	)	$	197	$	568	$	(7	)	$	561
Selling and administrative expenses	$	74		$	—		$	74	$	221	$	2		$	223
Research and technical expenses	$	17		$	—		$	17	$	48	$	—		$	48
Income (loss) from operations	$	109		$	(3	)	$	106	$	45	$	(9	)	$	36
Other income (expense)	$	(2	)	$	3		$	1	$	4	$	9		$	13

Employee Benefit Plans (Tables)

9 Months Ended

Jun. 30, 2019

Compensation And Retirement Disclosure [Abstract]

Net Periodic Defined Benefit Pension and Other Postretirement Benefit Costs

Net periodic defined benefit pension and other postretirement benefit costs include the following:

	Three Months Ended June 30
	2019						2018						2019					2018
	Pension Benefits												Postretirement Benefits
	U.S.			Foreign			U.S.			Foreign			U.S.			Foreign		U.S.			Foreign
	(In millions)
Service cost	$	1		$	2		$	1		$	2		$	—		$	—	$	—		$	—
Interest cost		1			1			1			2			—			—		1			—
Expected return on plan assets		(2	)		(2	)		(3	)		(4	)		—			—		—			—
Amortization of prior service cost (credit)		—			2			—			—			—			—		(1	)		—
Amortization of actuarial loss		—			1			—			1			(1	)		—		—			—
Settlement and curtailment gain		—			—			—			—			—			—		—			—
Net periodic benefit (credit) cost	$	—		$	4		$	(1	)	$	1		$	(1	)	$	—	$	—		$	—

	Nine Months Ended June 30
	2019						2018						2019					2018
	Pension Benefits												Postretirement Benefits
	U.S.			Foreign			U.S.			Foreign			U.S.			Foreign		U.S.			Foreign
	(In millions)
Service cost	$	1		$	5		$	1		$	6		$	—		$	—	$	—		$	—
Interest cost		4			4			3			6			1			—		1			—
Expected return on plan assets		(7	)		(8	)		(8	)		(11	)		—			—		—			—
Amortization of prior service cost (credit)		—			2			—			—			(1	)		—		(2	)		—
Amortization of actuarial loss		—			2			—			2			(1	)		—		—			—
Settlement and curtailment gain		—			(6	)		—			—			—			—		—			—
Net periodic benefit (credit) cost	$	(2	)	$	(1	)	$	(4	)	$	3		$	(1	)	$	—	$	(1	)	$	—

Purification Solutions Goodwill and Long-Lived Assets Impairment Charges (Tables)

9 Months Ended

Jun. 30, 2019

Goodwill And Intangible Assets Disclosure [Abstract]

Impairment Charges and Associated Deferred Tax Benefit

	Three Months Ended March 31, 2018
	(In millions)
Purification Solutions goodwill impairment charge	$	92
Purification Solutions long-lived assets impairment charge		162
Benefit for income taxes		(30	)
Impairment charges, after tax	$	224

Goodwill and Intangible Assets (Tables)

9 Months Ended

Jun. 30, 2019

Goodwill And Intangible Assets Disclosure [Abstract]

Schedule of Goodwill Balances

The carrying amount of goodwill attributable to each reportable segment with goodwill balances and the changes in those balances during the nine month period ended June 30, 2019 are as follows:

	Reinforcement Materials			Performance Chemicals		Total
	(In millions)
Balance at September 30, 2018	$	52		$	41	$	93
Foreign currency impact		(1	)		—		(1	)
Balance at June 30, 2019	$	51		$	41	$	92

Schedule of Intangible Assets

The following table provides information regarding the Company’s intangible assets:

	June 30, 2019							September 30, 2018
	Gross Carrying Value		Accumulated Amortization			Net Intangible Assets		Gross Carrying Value		Accumulated Amortization			Net Intangible Assets
	(In millions)
Intangible assets with finite lives
Developed technologies	$	51	$	(4	)	$	47	$	52	$	(2	)	$	50
Trademarks		8		—			8		8		—			8
Customer relationships		58		(13	)		45		51		(11	)		40
Total intangible assets	$	117	$	(17	)	$	100	$	111	$	(13	)	$	98

Accumulated Other Comprehensive Income (Loss) (Tables)

9 Months Ended

Jun. 30, 2019

Equity [Abstract]

Changes in Each Component of AOCI, Net of Tax

Changes in each component of AOCI, net of tax, were as follows:

	Currency Translation Adjustment			Unrealized Gains on Investments		Pension and Other Postretirement Benefit Liability Adjustments			Total
	(In millions)
Balance at September 30, 2018, attributable to Cabot Corporation	$	(266	)	$	1	$	(52	)	$	(317	)
Other comprehensive income (loss) before reclassifications		(24	)		—		30			6
Amounts reclassified from AOCI		(1	)		—		(8	)		(9	)
Less: Other comprehensive income (loss) attributable to noncontrolling interests		—			—		—			—
Balance at December 31, 2018, attributable to Cabot Corporation	$	(291	)	$	1	$	(30	)	$	(320	)
Other comprehensive income (loss) before reclassifications		8			—		1			9
Amounts reclassified from AOCI		(1	)		—		(1	)		(2	)
Less: Other comprehensive income (loss) attributable to noncontrolling interests		2			—		—			2
Balance at March 31, 2019, attributable to Cabot Corporation		(286	)		1		(30	)		(315	)
Other comprehensive income (loss) before reclassifications		1			—		(1	)		—
Amounts reclassified from AOCI		—			—		2			2
Less: Other comprehensive income (loss) attributable to noncontrolling interests		(2	)		—		—			(2	)
Balance at June 30, 2019, attributable to Cabot Corporation	$	(283	)	$	1	$	(29	)	$	(311	)

Amounts Reclassified Out of AOCI

The amounts reclassified out of AOCI and into the Consolidated Statements of Operations in the three and nine months ended June 30, 2019 and 2018 were as follows:

	Affected Line Item in the Consolidated	Three Months Ended June 30						Nine Months Ended June 30
	Statements of Operations	2019			2018			2019			2018
		(In millions)
Derivatives: net investment hedges
(Gains) losses reclassified to interest expense	Interest expense	$	(1	)	$	(2	)	$	(4	)	$	(4	)
(Gains) losses excluded from effectiveness testing and amortized to interest expense	Interest expense	$	—		$	—		$	1		$	—
Pension and other postretirement benefit liability adjustment
Amortization of actuarial losses	Net Periodic Benefit Cost - see Note E for details		—			1			1			2
Amortization of prior service cost (credit)	Net Periodic Benefit Cost - see Note E for details		2			(1	)		1			(2	)
Settlement and curtailment gain	Net Periodic Benefit Cost - see Note E for details		—			—			(6	)		—
Total before tax			1			(2	)		(7	)		(4	)
Tax impact	Provision (benefit) for income taxes		1			1			(2	)		1
Total after tax		$	2		$	(1	)	$	(9	)	$	(3	)

Commitments and Contingencies (Tables)

9 Months Ended

Jun. 30, 2019

Commitments And Contingencies Disclosure [Abstract]

Schedule of Components of Purchase Commitments

	Payments Due by Fiscal Year
	Remainder of Fiscal 2019		2020		2021		2022		2023		Thereafter		Total
	(In millions)
Reinforcement Materials	$	100	$	251	$	139	$	131	$	123	$	1,584	$	2,328
Performance Chemicals		27		63		59		57		34		510		750
Purification Solutions		1		6		1		—		—		—		8
Total	$	128	$	320	$	199	$	188	$	157	$	2,094	$	3,086

Income Tax (Tables)

9 Months Ended

Jun. 30, 2019

Effective Tax Rate

	Three Months Ended June 30						Nine Months Ended June 30
	2019			2018			2019			2018
	(Dollars in millions)
(Provision) benefit for income taxes	$	(30	)	$	4		$	(43	)	$	(194	)
Effective tax rate		43	%		(3	)%		23	%		1202	%

Earnings Per Share (Tables)

9 Months Ended

Jun. 30, 2019

Components of Basic and Diluted Earnings (Loss) Per Common Share (EPS)

The following tables summarize the components of the basic and diluted earnings (loss) per common share (“EPS”) computations:

	Three Months Ended June 30				Nine Months Ended June 30
	2019		2018		2019		2018
	(In millions, except per share amounts)
Basic EPS:
Net income (loss) attributable to Cabot Corporation	$	32	$	88	$	124	$	(207	)
Less: Undistributed earnings allocated to participating securities⁽¹⁾		—		—		—		—
Earnings (loss) allocated to common shareholders (numerator)	$	32	$	88	$	124	$	(207	)

Weighted average common shares and participating securities outstanding		59.0		62.5		59.9		62.5
Less: Participating securities⁽¹⁾		0.8		0.7		0.8		0.7
Adjusted weighted average common shares (denominator)		58.2		61.8		59.1		61.8

Earnings (loss) per common share - basic:	$	0.55	$	1.41	$	2.08	$	(3.36	)

Diluted EPS:
Earnings (loss) allocated to common shareholders	$	32	$	88	$	124	$	(207	)
Plus: Earnings (loss) allocated to participating securities		—		1		1		1
Less: Adjusted earnings allocated to participating securities⁽²⁾		—		1		1		1
Earnings (loss) allocated to common shareholders (numerator)	$	32	$	88	$	124	$	(207	)

Adjusted weighted average common shares outstanding		58.2		61.8		59.1		61.8
Effect of dilutive securities:
Common shares issuable⁽³⁾		0.2		0.5		0.1		—
Adjusted weighted average common shares (denominator)		58.4		62.3		59.2		61.8

Earnings (loss) per common share - diluted:	$	0.55	$	1.40	$	2.08	$	(3.36	)

⁽¹⁾	^{Participating securities consist of shares underlying outstanding and achieved performance-based restricted stock units issued during and after fiscal 2017 and all unvested time-based restricted stock units.}

Calculation of Undistributed Earnings

	Three Months Ended June 30				Nine Months Ended June 30
	2019		2018		2019		2018
	(In millions)
Calculation of undistributed earnings (loss):
Net income (loss) attributable to Cabot Corporation	$	32	$	88	$	124	$	(207	)
Less: Dividends declared on common stock		20		21		60		60
Undistributed earnings (loss)	$	12	$	67	$	64	$	(267	)

Allocation of undistributed earnings (loss):
Undistributed earnings (loss) allocated to common shareholders	$	12	$	67	$	64	$	(267	)
Undistributed earnings (loss)	$	12	$	67	$	64	$	(267	)

⁽²⁾	Undistributed earnings are adjusted for the assumed distribution of dividends to the dilutive securities, which are described in (3) below, and then reallocated to participating securities.

⁽³⁾

Restructuring (Tables)

9 Months Ended

Jun. 30, 2019

Restructuring And Related Activities [Abstract]

Recorded Restructuring Activities

Cabot’s restructuring activities were recorded in the Consolidated Statements of Operations in the three and nine months ended June 30, 2019 and 2018 as follows:

	Three Months Ended June 30				Nine Months Ended June 30
	2019		2018		2019		2018
	(In millions)
Cost of sales	$	2	$	1	$	7	$	(8	)
Selling and administrative expenses		2		—		8		1
Total	$	4	$	1	$	15	$	(7	)

Restructuring Activities and Related Reserves

Details of all restructuring activities and the related reserves during the three and nine months ended June 30, 2019 were as follows:

	Severance and Employee Benefits			Environmental Remediation		Asset Impairment and Accelerated Depreciation			Other			Total
	(In millions)
Reserve at September 30, 2018	$	1		$	4	$	—		$	—		$	5
Charges (gain)		7			—		1			1			9
Costs charged against assets / (liabilities)		—			1		(1	)		—			—
Cash paid		(1	)		—		—			(1	)		(2	)
Reserve at December 31, 2018		7			5		—			—			12
Charges (gain)		1			—		—			1			2
Costs charged against assets / (liabilities)		—			—		—			—			—
Cash paid		(1	)		—		—			(1	)		(2	)
Reserve at March 31, 2019		7			5		—			—			12
Charges (gain)		2			—		1			1			4
Costs charged against assets / (liabilities)		—			—		(1	)					(1	)
Cash paid		(2	)		—		—			(1	)		(3	)
Reserve at June 30, 2019	$	7		$	5	$	—		$	—		$	12

Derivatives (Tables)

9 Months Ended

Jun. 30, 2019

Derivative Instruments And Hedging Activities Disclosure [Abstract]

Details of Derivatives Held to Manage Foreign Currency Risk

The following table provides details of the derivatives held as of June 30, 2019 and September 30, 2018 to manage foreign currency risk.

		Notional Amount
Description	Borrowing	June 30, 2019	September 30, 2018	Hedge Designation
Cross-Currency Swaps	3.4% Notes	USD 250 million swapped to EUR 223 million	USD 250 million swapped to EUR 223 million	Net investment
Forward Foreign Currency Contracts ⁽¹⁾	N/A	USD 28 million	USD 18 million	No designation

⁽¹⁾	Cabot’s forward foreign exchange contracts are denominated in the Canadian dollar, Indonesian rupiah and Czech koruna.

Summary Impact of Cross-currency Swaps to AOCI and Consolidated Statements of Operations

The following tables summarize the impact of the cross-currency swaps to AOCI and the Consolidated Statements of Operations:

	Three Months Ended June 30
	2019			2018		2019			2018			2019		2018
Description	Gain/(Loss) Recognized in AOCI					(Gain)/Loss Reclassified from AOCI into Interest Expense in the Consolidated Statements of Operations						(Gain)/Loss Recognized in Interest Expense in the Consolidated Statements of Operations (Amount Excluded from Effectiveness Testing)
	(In millions)
Cross-currency swaps	$	(1	)	$	10	$	(1	)	$	(2	)	$	—	$	—

	Nine Months Ended June 30
	2019		2018			2019			2018			2019		2018
Description	Gain/(Loss) Recognized in AOCI					(Gain)/Loss Reclassified from AOCI into Interest Expense in the Consolidated Statements of Operations						(Gain)/Loss Recognized in Interest Expense in the Consolidated Statements of Operations (Amount Excluded from Effectiveness Testing)
	(In millions)
Cross-currency swaps	$	12	$	(3	)	$	(4	)	$	(4	)	$	1	$	1

+ References

+ Details

Financial Information by Segment (Tables)

9 Months Ended

Jun. 30, 2019

Schedule of Performance Segment

The net sales from each of these businesses for the three and nine months ended June 30, 2019 and 2018 were as follows:

	Three Months Ended June 30				Nine Months Ended June 30
	2019		2018		2019		2018
	(In millions)
Performance Additives	$	172	$	188	$	518	$	524
Formulated Solutions		79		86		218		247
Performance Chemicals	$	251	$	274	$	736	$	771

Financial Information by Reportable Segment

Financial information by reportable segment is as follows:

	Reinforcement Materials		Performance Chemicals		Purification Solutions			Specialty Fluids			Segment Total		Unallocated and Other⁽¹⁾			Consolidated Total
	(In millions)
Three Months Ended June 30, 2019
Revenues from external customers⁽²⁾	$	461	$	251	$	73		$	13		$	798	$	47		$	845
Income (loss) from continuing operations before income taxes⁽³⁾	$	72	$	37	$	1		$	2		$	112	$	(43	)	$	69

Three Months Ended June 30, 2018
Revenues from external customers⁽²⁾	$	466	$	274	$	70		$	12		$	822	$	32		$	854
Income (loss) from continuing operations before income taxes⁽³⁾	$	74	$	56	$	(6	)	$	3		$	127	$	(32	)	$	95

Nine Months Ended June 30, 2019
Revenues from external customers⁽²⁾	$	1,363	$	736	$	210		$	56		$	2,365	$	145		$	2,510
Income (loss) from continuing operations before income taxes⁽³⁾	$	195	$	111	$	(1	)	$	24		$	329	$	(141	)	$	188

Nine Months Ended June 30, 2018
Revenues from external customers⁽²⁾	$	1,307	$	771	$	206		$	24		$	2,308	$	84		$	2,392
Income (loss) from continuing operations before income taxes⁽³⁾	$	215	$	160	$	(6	)	$	(2	)	$	367	$	(351	)	$	16

⁽¹⁾	Unallocated and Other includes certain items and eliminations necessary to reflect management’s reporting of operating segment results. These items are reflective of the segment reporting presented to the CODM.

⁽²⁾

	Three Months Ended June 30						Nine Months Ended June 30
	2019			2018			2019			2018
	(In millions)
Royalties, the impact of unearned revenue, the removal of 100% of the sales of an equity method affiliate and discounting charges for certain notes receivable	$	(3	)	$	(2	)	$	(9	)	$	(10	)
Shipping and handling fees		32			34			96			94
By-product sales^(a)		18			—			58			—
Total	$	47		$	32		$	145		$	84

^(a)

⁽³⁾

	Three Months Ended June 30						Nine Months Ended June 30
	2019			2018			2019			2018
	(In millions)
Interest expense	$	(14	)	$	(14	)	$	(43	)	$	(41	)
Certain items^(a)
Specialty Fluids loss on sale and asset impairment charge (Note D)		(8	)		—			(28	)		—
Equity affiliate investment impairment charge (Note O)		—			—			(11	)		—
Purification Solutions goodwill and long-lived assets impairment charge (Note F)		—			—			—			(254	)
Inventory reserve adjustment (Note F)		—			—			—			(13	)
Global restructuring activities (Note L)		(4	)		(1	)		(15	)		7
Legal and environmental matters and reserves		—			—			(1	)		(6	)
Gains (losses) on sale of investments		—			—			—			10
Acquisition and integration-related charges		(1	)		—			(5	)		(1	)
Other		(1	)		(2	)		(1	)		(3	)
Total certain items, pre-tax		(14	)		(3	)		(61	)		(260	)
Unallocated corporate costs^(b)		(14	)		(15	)		(39	)		(45	)
General unallocated income (expense)^(c)		—			—			3			(3	)
Less: Equity in earnings of affiliated companies, net of tax^(d)		1			—			1			2
Total	$	(43	)	$	(32	)	$	(141	)	$	(351	)

^(a)

^(b)

Unallocated corporate costs are costs that are not controlled by the segments and primarily benefit corporate interests.

^(c)

^(d)

Revenues from External Customers by Geographic Region

	Three Months Ended June 30, 2019
	Reinforcement Materials		Performance Chemicals		Purification Solutions		Specialty Fluids		Consolidated Total
	(In millions)
Americas	$	179	$	69	$	33	$	2	$	283
Asia Pacific		191		93		10		—		294
Europe, Middle East and Africa		91		89		30		11		221
Segment revenues from external customers		461		251		73		13		798
Unallocated and other										47
Net sales and other operating revenues									$	845

	Nine Months Ended June 30, 2019
	Reinforcement Materials		Performance Chemicals		Purification Solutions		Specialty Fluids		Consolidated Total
	(In millions)
Americas	$	515	$	222	$	93	$	6	$	836
Asia Pacific		573		253		26		1		853
Europe, Middle East and Africa		275		261		91		49		676
Segment revenues from external customers		1,363		736		210		56		2,365
Unallocated and other										145
Net sales and other operating revenues									$	2,510

Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended		3 Months Ended		9 Months Ended
	Nov. 30, 2018	Nov. 30, 2013	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Significant Accounting Policies [Line Items]
Net sales and other operating revenues			$ 845.0	$ 854.0	$ 2,510.0	$ 2,392.0
Cost of sales			675.0	$ 657.0	1,996.0	$ 1,831.0
Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Significant Accounting Policies [Line Items]
Net sales and other operating revenues			(18.0)		(58.0)
Cost of sales			$ (18.0)		$ (58.0)
NHUMO [Member]
Significant Accounting Policies [Line Items]
Preferred stock repurchase amount	$ 25.0	$ 25.0
Dividend percentage of preferred stock		6.00%
Preferred stock dividend	$ 1.4
Minimum [Member]
Significant Accounting Policies [Line Items]
Payment terms					0 days
Minimum [Member] \| Buildings [Member]
Significant Accounting Policies [Line Items]
Useful life of property, plant and equipment					20 years
Minimum [Member] \| Machinery and Equipment [Member]
Significant Accounting Policies [Line Items]
Useful life of property, plant and equipment					10 years
Minimum [Member] \| Other Fixed Assets [Member]
Significant Accounting Policies [Line Items]
Useful life of property, plant and equipment					3 years
Maximum [Member]
Significant Accounting Policies [Line Items]
Payment terms					90 days
Maximum [Member] \| Buildings [Member]
Significant Accounting Policies [Line Items]
Useful life of property, plant and equipment					25 years
Maximum [Member] \| Machinery and Equipment [Member]
Significant Accounting Policies [Line Items]
Useful life of property, plant and equipment					25 years
Maximum [Member] \| Other Fixed Assets [Member]
Significant Accounting Policies [Line Items]
Useful life of property, plant and equipment					25 years

Significant Accounting Policies - Schedule of Change Retrospectively to Prior Periods Consolidated Statements of Operations (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Significant Accounting Policies [Line Items]
Cost of sales	$ 675	$ 657	$ 1,996	$ 1,831
Gross profit	170	197	514	561
Selling and administrative expenses	65	74	208	223
Research and technical expenses	16	17	47	48
Income (loss) from operations	$ 81	106	231	36
Other income (expense)		1	$ (6)	13
Change in Method of Accounting for Inventories from Last-in, First-out Method to First-in, First-out Method [Member]
Significant Accounting Policies [Line Items]
Cost of sales		657		1,831
Gross profit		197		561
Selling and administrative expenses		74		223
Research and technical expenses		17		48
Income (loss) from operations		106		36
Other income (expense)		1		13
Change in Method of Accounting for Inventories from Last-in, First-out Method to First-in, First-out Method [Member] \| As Originally Reported [Member]
Significant Accounting Policies [Line Items]
Cost of sales		654		1,824
Gross profit		200		568
Selling and administrative expenses		74		221
Research and technical expenses		17		48
Income (loss) from operations		109		45
Other income (expense)		(2)		4
Change in Method of Accounting for Inventories from Last-in, First-out Method to First-in, First-out Method [Member] \| Effect of Change [Member]
Significant Accounting Policies [Line Items]
Cost of sales		3		7
Gross profit		(3)		(7)
Selling and administrative expenses				2
Income (loss) from operations		(3)		(9)
Other income (expense)		$ 3		$ 9

Acquisitions - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended			3 Months Ended	9 Months Ended
	Jun. 30, 2019	Sep. 30, 2018	Nov. 30, 2017	Jun. 30, 2019	Jun. 30, 2019
Tech Blend [Member]
Business Acquisition [Line Items]
Acquisition date			Nov. 30, 2017
Business acquisition, purchase price			$ 65
NSCC Carbon (Jiangsu) Co. Ltd [Member]
Business Acquisition [Line Items]
Acquisition date		Sep. 30, 2018
Expected Production commencement year		2021
Transition related costs				$ 1	$ 4
NSCC Carbon (Jiangsu) Co. Ltd [Member] \| Accounts Payable and Accrued Liabilities [Member]
Business Acquisition [Line Items]
Business acquisition, purchase price		$ 8
Other Acquisition [Member] \| Intangible Assets [Member]
Business Acquisition [Line Items]
Business acquisition, purchase price	$ 3

Sale of the Specialty Fluids Business - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2019
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total proceeds from sale of Specialty Fluids business				$ 130
Proceeds held in escrow				5
Specialty Fluids loss on sale and asset impairment (Note D)		$ 8		$ 28
Sinomine (Hong Kong) Rare Metals Resources Co. Limited [Member] \| Specialty Fluids [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total proceeds from sale of Specialty Fluids business	$ 133
Amount of cash transferred	2
Proceeds held in escrow	5
Pre-tax loss on sale of Specialty Fluids business		$ 8
Specialty Fluids loss on sale and asset impairment (Note D)			$ 20
Additional cash royalties for lithium products	$ 5
Royalty payable term	10 years

Employee Benefit Plans - Net Periodic Defined Benefit Pension and Other Postretirement Benefit Costs (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Pension Benefits [Member] \| U.S. Defined Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1	$ 1	$ 1	$ 1
Interest cost	1	1	4	3
Expected return on plan assets	(2)	(3)	(7)	(8)
Net periodic benefit (credit) cost		(1)	(2)	(4)
Pension Benefits [Member] \| Foreign Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	2	2	5	6
Interest cost	1	2	4	6
Expected return on plan assets	(2)	(4)	(8)	(11)
Amortization of prior service cost (credit)	2		2
Amortization of actuarial loss	1	1	2	2
Settlement and curtailment gain			(6)
Net periodic benefit (credit) cost	4	1	(1)	3
Postretirement Benefits [Member] \| U.S. Defined Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost		1	1	1
Amortization of prior service cost (credit)		$ (1)	(1)	(2)
Amortization of actuarial loss	(1)		(1)
Net periodic benefit (credit) cost	$ (1)		$ (1)	$ (1)

Employee Benefit Plans - Additional Information (Detail) $ in Millions	3 Months Ended
	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($) Plan
Defined Benefit Plan Disclosure [Line Items]
Number of foreign defined benefit plans \| Plan		1
Reduction in net pension obligations		$ (30)
Pre-tax gain from net pension obligations		6
Settlement credit		5
Plan assets remain to be transferred.	$ 1
Additional charge on defined benefit plan actuarial gain (loss)		$ 3
Pay out period for additional charge payment		5 years
Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Curtailment credit		$ 1
U.K. Plan [Member] \| Other Income (Expense) [Member]
Defined Benefit Plan Disclosure [Line Items]
Charge on effect of accued benefit	$ 2

Purification Solutions Goodwill and Long-Lived Assets Impairment Charges - Impairment Charges and Associated Tax Benefit (Detail) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Goodwill And Intangible Assets [Line Items]
Purification Solutions goodwill impairment charge					$ 92
Purification Solutions long-lived assets impairment charge					162
Benefit for income taxes				$ (20)	$ 131
Impairment charges, after tax	$ 8			$ 28
Purification Solutions [Member]
Goodwill And Intangible Assets [Line Items]
Purification Solutions goodwill impairment charge		$ 92	$ 92
Purification Solutions long-lived assets impairment charge			162
Benefit for income taxes			(30)
Impairment charges, after tax			$ 224

Purification Solutions Goodwill and Long-Lived Assets Impairment Charges - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2018
Goodwill And Intangible Assets [Line Items]
Goodwill impairment charge			$ 92
Long-lived assets impairment charge			$ 162
Assets impairment charges income tax benefit	$ 30
Reducing of impairment charges and restructuring costs	1
Purification Solutions [Member]
Goodwill And Intangible Assets [Line Items]
Goodwill impairment charge	$ 92	$ 92
Lower of cost or market reserve inventory		13
Long-lived assets impairment charge		162
Impairment charges of definite lived Intangible assets		64
Purification Solutions [Member] \| Property, Plant and Equipment [Member]
Goodwill And Intangible Assets [Line Items]
Long-lived assets impairment charge		$ 98

Goodwill and Intangible Assets - Schedule of Goodwill Balances (Detail) $ in Millions	9 Months Ended
	Jun. 30, 2019 USD ($)
Goodwill And Intangible Assets [Line Items]
Beginning balance	$ 93
Foreign currency impact	(1)
Ending balance	92
Reinforcement Materials [Member]
Goodwill And Intangible Assets [Line Items]
Beginning balance	52
Foreign currency impact	(1)
Ending balance	51
Performance Chemicals [Member]
Goodwill And Intangible Assets [Line Items]
Beginning balance	41
Ending balance	$ 41

Goodwill and Intangible Assets - Schedule of Intangible Assets (Detail) - USD ($) $ in Millions	Jun. 30, 2019	Sep. 30, 2018
Finite Lived Intangible Assets [Line Items]
Gross Carrying Value, finite lives	$ 117	$ 111
Accumulated Amortization	(17)	(13)
Net Intangible Assets, finite lives	100	98
Developed Technologies [Member]
Finite Lived Intangible Assets [Line Items]
Gross Carrying Value, finite lives	51	52
Accumulated Amortization	(4)	(2)
Net Intangible Assets, finite lives	47	50
Trademarks [Member]
Finite Lived Intangible Assets [Line Items]
Gross Carrying Value, finite lives	8	8
Net Intangible Assets, finite lives	8	8
Customer Relationships [Member]
Finite Lived Intangible Assets [Line Items]
Gross Carrying Value, finite lives	58	51
Accumulated Amortization	(13)	(11)
Net Intangible Assets, finite lives	$ 45	$ 40

Goodwill and Intangible Assets - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Goodwill And Intangible Assets [Line Items]
Amortization expense estimated for year one	$ 6		$ 6
Amortization expense estimated for year two	6		6
Amortization expense estimated for year three	6		6
Amortization expense estimated for year four	6		6
Amortization expense estimated for year five	6		6
Cost of Sales, Selling and Administrative Expenses and Research and Technical Expenses [Member]
Goodwill And Intangible Assets [Line Items]
Amortization of intangible assets	$ 1	$ 1	$ 4	$ 6
Minimum [Member]
Goodwill And Intangible Assets [Line Items]
Useful life of intangible assets			12 years
Maximum [Member]
Goodwill And Intangible Assets [Line Items]
Useful life of intangible assets			25 years
Weighted Average [Member]
Goodwill And Intangible Assets [Line Items]
Useful life of intangible assets			19 years

Accumulated Other Comprehensive Income (Loss) - Changes in Each Component of AOCI, Net of Tax (Detail) - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	$ 1,232	$ 1,276	$ 1,279	$ 1,343	$ 1,279	$ 1,625
Ending Balance	1,226	1,232	1,276	1,289	1,226	1,289
Currency Translation Adjustment [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	(286)	(291)	(266)		(266)
Other comprehensive income (loss) before reclassifications	1	8	(24)
Amounts reclassified from AOCI		(1)	(1)
Less: Other comprehensive income (loss) attributable to noncontrolling interests	(2)	2
Ending Balance	(283)	(286)	(291)		(283)
Unrealized Gains on Investments [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	1	1	1		1
Ending Balance	1	1	1		1
Pension and Other Postretirement Benefit Liability Adjustments [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	(30)	(30)	(52)		(52)
Other comprehensive income (loss) before reclassifications	(1)	1	30
Amounts reclassified from AOCI	2	(1)	(8)
Ending Balance	(29)	(30)	(30)		(29)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	(315)	(320)	(317)	(209)	(317)	(259)
Other comprehensive income (loss) before reclassifications		9	6
Amounts reclassified from AOCI	2	(2)	(9)	(1)	(9)	(3)
Less: Other comprehensive income (loss) attributable to noncontrolling interests	(2)	2
Ending Balance	$ (311)	$ (315)	$ (320)	$ (313)	$ (311)	$ (313)

Accumulated Other Comprehensive Income (Loss) - Amounts Reclassified Out of AOCI (Detail) - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
(Gains) Losses Reclassified to Interest Expense [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	$ (1)			$ (2)	$ (4)	$ (4)
(Gains) Losses Excluded from Effectiveness Testing and Amortized to Interest Expense [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax					1
Amortization of Actuarial Losses [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax				1	1	2
Amortization of Prior Service Cost (Credit) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	2			(1)	1	(2)
Settlement and Curtailment Gain [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax					(6)
Accumulated Other Comprehensive Income (Loss) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	1			(2)	(7)	(4)
Tax impact	1			1	(2)	1
Total after tax	$ 2	$ (2)	$ (9)	$ (1)	$ (9)	$ (3)

Commitments and Contingencies - Schedule of Components of Purchase Commitments (Detail) $ in Millions	Jun. 30, 2019 USD ($)
Long-term Purchase Commitment [Line Items]
Payments Due by Remainder of Fiscal Year 2019	$ 128
Payments Due by Fiscal Year 2020	320
Payments Due by Fiscal Year 2021	199
Payments Due by Fiscal Year 2022	188
Payments Due by Fiscal Year 2023	157
Payments Due Thereafter	2,094
Payments Due, Total	3,086
Reinforcement Materials [Member]
Long-term Purchase Commitment [Line Items]
Payments Due by Remainder of Fiscal Year 2019	100
Payments Due by Fiscal Year 2020	251
Payments Due by Fiscal Year 2021	139
Payments Due by Fiscal Year 2022	131
Payments Due by Fiscal Year 2023	123
Payments Due Thereafter	1,584
Payments Due, Total	2,328
Performance Chemicals [Member]
Long-term Purchase Commitment [Line Items]
Payments Due by Remainder of Fiscal Year 2019	27
Payments Due by Fiscal Year 2020	63
Payments Due by Fiscal Year 2021	59
Payments Due by Fiscal Year 2022	57
Payments Due by Fiscal Year 2023	34
Payments Due Thereafter	510
Payments Due, Total	750
Purification Solutions [Member]
Long-term Purchase Commitment [Line Items]
Payments Due by Remainder of Fiscal Year 2019	1
Payments Due by Fiscal Year 2020	6
Payments Due by Fiscal Year 2021	1
Payments Due, Total	$ 8

Commitments and Contingencies - Additional Information (Detail) $ in Millions	9 Months Ended
	Jun. 30, 2019 USD ($) claimants	Jun. 30, 2018 USD ($)	Sep. 30, 2018 USD ($) claimants
Respirator Liabilities [Member]
Loss Contingencies [Line Items]
Number of claimants \| claimants	36,000		35,000
Cash payments for respirator reserves	$ 4	$ 1
Respirator reserve	21		$ 25
Environmental Matters [Member]
Loss Contingencies [Line Items]
Reserved for environmental matters	14		$ 15
Cash payments for environmental reserves	$ 1	$ 2

Income Tax - Effective Tax Rate (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Tax Disclosure [Abstract]
(Provision) benefit for income taxes	$ (30)	$ 4	$ (43)	$ (194)
Effective tax rate	43.00%	(3.00%)	23.00%	1202.00%

Income Tax - Additional Information (Detail) - USD ($)		3 Months Ended				9 Months Ended
Income Tax - Additional Information (Detail) - USD ($)	Jan. 01, 2018	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2019	Jun. 30, 2018
Income Taxes [Line Items]
Net discrete tax (benefits) charges related to tax settlements		$ 13,000,000		$ (23,000,000)		$ (11,000,000)	$ 139,000,000
Net tax expense from impact of the Act		17,000,000		(4,000,000)		$ 0	185,000,000
Income tax rate	21.00%				35.00%
Percentage of dividend received deduction for foreign dividends						100.00%
Benefit for income taxes for deemed repatriation			$ 17,000,000
Additional tax expense due to reversal		17,000,000
Uncertain tax positions from expirations of statute of limitations		$ 1,000,000		$ 0		$ 9,000,000	$ 2,000,000
Earliest Tax Year [Member] \| United States Internal Revenue Service (IRS) [Member]
Income Taxes [Line Items]
Tax years remain subject to examination						2014
Latest Tax Year [Member] \| United States Internal Revenue Service (IRS) [Member]
Income Taxes [Line Items]
Tax years remain subject to examination						2017
State Tax Authorities [Member] \| Earliest Tax Year [Member]
Income Taxes [Line Items]
Tax years remain subject to examination						2005
State Tax Authorities [Member] \| Latest Tax Year [Member]
Income Taxes [Line Items]
Tax years remain subject to examination						2017
Non-U.S. Jurisdictions [Member] \| Earliest Tax Year [Member]
Income Taxes [Line Items]
Tax years remain subject to examination						2002
Non-U.S. Jurisdictions [Member] \| Latest Tax Year [Member]
Income Taxes [Line Items]
Tax years remain subject to examination						2017

Earnings Per Share - Components of Basic and Diluted Earnings (Loss) Per Common Share (EPS) (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Basic EPS:
Net income (loss) attributable to Cabot Corporation	$ 32	$ 88	$ 124	$ (207)
Earnings (loss) allocated to common shareholders (numerator)	$ 32	$ 88	$ 124	$ (207)
Weighted average common shares and participating securities outstanding	59.0	62.5	59.9	62.5
Less: Participating securities	0.8	0.7	0.8	0.7
Adjusted weighted average common shares (denominator)	58.2	61.8	59.1	61.8
Earnings (loss) per common share - basic:	$ 0.55	$ 1.41	$ 2.08	$ (3.36)
Diluted EPS:
Earnings (loss) allocated to common shareholders	$ 32	$ 88	$ 124	$ (207)
Plus: Earnings (loss) allocated to participating securities		1	1	1
Less: Adjusted earnings allocated to participating securities		1	1	1
Earnings (loss) allocated to common shareholders (numerator)	$ 32	$ 88	$ 124	$ (207)
Adjusted weighted average common shares outstanding	58.2	61.8	59.1	61.8
Common shares issuable	0.2	0.5	0.1
Adjusted weighted average common shares (denominator)	58.4	62.3	59.2	61.8
Earnings (loss) per common share - diluted:	$ 0.55	$ 1.40	$ 2.08	$ (3.36)

Earnings Per Share - Calculation of Undistributed Earnings (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Earnings Per Share [Abstract]
Net income (loss) attributable to Cabot Corporation	$ 32	$ 88	$ 124	$ (207)
Less: Dividends declared on common stock	20	21	60	60
Undistributed earnings (loss)	12	67	64	(267)
Undistributed earnings (loss) allocated to common shareholders	$ 12	$ 67	$ 64	$ (267)

Net Investment Hedge [Member] | Cross Currency Swaps [Member]

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Earnings Per Share - Components of Basic and Diluted Earnings (Loss) Per Common Share (EPS) (Parenthetical) (Detail) - shares	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Earnings Per Share [Abstract]
Antidilutive securities excluded from computation of earnings per share	983,081	260,630	945,246	799,163

Restructuring - Recorded Restructuring Activities (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Restructuring Cost And Reserve [Line Items]
Restructuring reserve, period expense	$ 4	$ 1	$ 15	$ (7)
Cost of Sales [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring reserve, period expense	2	$ 1	7	(8)
Selling and Administrative Expenses [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring reserve, period expense	$ 2		$ 8	$ 1

Restructuring - Restructuring Activities and Related Reserves (Detail) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2019
Restructuring Cost And Reserve [Line Items]
Reserve balance	$ 12	$ 12	$ 5	$ 5
Charges (gain)	4	2	9
Costs charged against assets / (liabilities)	(1)
Cash paid	(3)	(2)	(2)
Reserve balance	12	12	12	12
Severance and Employee Benefits [Member]
Restructuring Cost And Reserve [Line Items]
Reserve balance	7	7	1	1
Charges (gain)	2	1	7
Cash paid	(2)	(1)	(1)
Reserve balance	7	7	7	7
Environmental Remediation [Member]
Restructuring Cost And Reserve [Line Items]
Reserve balance	5	5	4	4
Costs charged against assets / (liabilities)			1
Reserve balance	5	5	5	$ 5
Asset Impairment and Accelerated Depreciation [Member]
Restructuring Cost And Reserve [Line Items]
Charges (gain)	1		1
Costs charged against assets / (liabilities)	(1)		(1)
Other [Member]
Restructuring Cost And Reserve [Line Items]
Charges (gain)	1	1	1
Cash paid	$ (1)	$ (1)	$ (1)

Restructuring - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended					9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Restructuring Cost And Reserve [Line Items]
Restructuring Charges		$ 4	$ 2	$ 9
Proceeds from sale of land							$ 13
Purification Solutions Transformation Plan [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring Charges						$ 9
Severance costs						8
Professional fees						1
Restructuring charges expected remainder of fiscal year and thereafter		1				1
Restructuring cash payments of fiscal year		2				4
Accrued severance charges		5				$ 5
Purification Solutions Transformation Plan [Member] \| Maximum [Member]
Restructuring Cost And Reserve [Line Items]
Severance costs		$ 1
Purification Solutions Transformation Plan [Member] \| Scenario, Forecast [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring cash payments through the reminder of fiscal 2019 and thereafter	$ 6
Operational Restructuring [Member] \| Merak, Indonesia [Member] \| Cost of Sales [Member]
Restructuring Cost And Reserve [Line Items]
Proceeds from sale of land					$ 13
Net benefit to earnings					$ 11

Financial Instruments and Fair Value Measurements - Additional Information (Detail)	1 Months Ended	9 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	May 31, 2019 EUR (€)	Sep. 30, 2018 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, liabilities, Level 2 to Level 1 transfers, amount	$ 0	$ 0	$ 0
Fair value, assets, Level 2 to Level 1 transfers, amount	0	0	0
Fair value, liabilities, Level 1 to Level 2 transfers, amount	0	0	0
Fair value, assets, Level 1 to Level 2 transfers, amount	0	0	0
Fair value, assets, transfers into Level 3, amount		0	0
Fair value, assets, transfers out of Level 3, amount		0	0
Fair value, liabilities, transfers into Level 3, amount		0	0
Fair value, liabilities, transfers out of Level 3, amount		0	$ 0
Fair value of long-term debt	1,077,000,000	$ 1,077,000,000			$ 742,000,000
4% Notes mature on 2029 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt issuance cost	3,000,000
Registered unsecured notes issued	$ 300,000,000
Debt Instrument, interest rate	4.00%	4.00%
Debt maturity date	Jul. 01, 2029
Proceeds from issuance of unsecured notes	$ 296,000,000
Debt instrument, discount	1,000,000	$ 1,000,000
Revolving Credit Agreement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Revolving credit facility, committed amount \| €				€ 300,000,000
Credit agreement, available borrowing	294,000,000	$ 294,000,000
Debt instrument, maturity date		May 22, 2024
Debt issuance cost		$ 1,000,000
Unsecured Revolving Credit Agreement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Revolving credit facility, committed amount	1,000,000,000	1,000,000,000
Fixed Rate Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value of long-term debt	1,018,000,000	1,018,000,000			747,000,000
Significant Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets in the Consolidated Balance Sheets	11,000,000	11,000,000			11,000,000
Significant Observable Inputs (Level 2) [Member] \| Foreign Currency Risks [Member] \| Prepaid Expenses and Other Current Assets and Other Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets (liabilities), at fair Value, net	$ (8,000,000)	$ (8,000,000)			$ (18,000,000)

Derivatives - Details of Derivatives Held to Manage Foreign Currency Risk (Detail)

Jun. 30, 2019

USD ($)

Jun. 30, 2019

EUR (€)

Sep. 30, 2018

USD ($)

Sep. 30, 2018

EUR (€)

Net Investment Hedge [Member] | 3.4% Notes [Member] | Cross Currency Swaps [Member]

$ 250,000,000

Net Investment Hedge [Member] | 3.4% Notes [Member] | Fixed Rate Debt [Member] | Cross Currency Swaps [Member]

$ 250,000,000

€ 223,000,000

$ 250,000,000

€ 223,000,000

Interest rate on borrowing

3.40%

Not Designated as Hedging Instrument [Member] | Forward Foreign Currency Contracts [Member]