Basis of Presentation	6 Months Ended
Mar. 31, 2019
Accounting Policies [Abstract]
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 March 31, 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. In October 2018, the Company realigned its business reporting structure under the Performance Chemicals segment and now combines the specialty carbons, fumed metal oxides and aerogel product lines into the Performance Additives business, and the 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”). In light of this impending transaction, the assets and liabilities of the Specialty Fluids business are included in Assets held for sale and Liabilities held for sale in the Consolidated Balance Sheet as of March 31, 2019. The sale of the Specialty Fluids business does not meet the criteria to be reported as a discontinued operation and, therefore, prior periods’ consolidated financial statements and disclosures have not been recast. Refer to Note D for further details regarding the pending sale of the Specialty Fluids business.

Basis of Presentation

6 Months Ended

Mar. 31, 2019

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 March 31, 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.

In October 2018, the Company realigned its business reporting structure under the Performance Chemicals segment and now combines the specialty carbons, fumed metal oxides and aerogel product lines into the Performance Additives business, and the 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”). In light of this impending transaction, the assets and liabilities of the Specialty Fluids business are included in Assets held for sale and Liabilities held for sale in the Consolidated Balance Sheet as of March 31, 2019. The sale of the Specialty Fluids business does not meet the criteria to be reported as a discontinued operation and, therefore, prior periods’ consolidated financial statements and disclosures have not been recast. Refer to Note D for further details regarding the pending sale of the Specialty Fluids business.