Basis of Presentation	9 Months Ended
Jun. 30, 2018
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, 2017 (“2017 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, 2018 and 2017. 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, 2017, the Company changed its method of accounting for its U.S. carbon black inventories from the last-in, first-out (“LIFO”) method to the first-in, first-out (“FIFO”) method. The Company applied this change retrospectively to all prior periods presented, which is discussed in further detail in Note B. As discussed in Note C, in November 2017, the Company acquired all of the issued and outstanding shares of 8755329 Canada Inc. (“Tech Blend”), a North American producer of black masterbatches. The financial position, results of operations and cash flows of Tech Blend are included in the Company’s consolidated financial statements from the date of acquisition.

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, 2017 (“2017 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, 2018 and 2017. 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, 2017, the Company changed its method of accounting for its U.S. carbon black inventories from the last-in, first-out (“LIFO”) method to the first-in, first-out (“FIFO”) method. The Company applied this change retrospectively to all prior periods presented, which is discussed in further detail in Note B.

As discussed in Note C, in November 2017, the Company acquired all of the issued and outstanding shares of 8755329 Canada Inc. (“Tech Blend”), a North American producer of black masterbatches. The financial position, results of operations and cash flows of Tech Blend are included in the Company’s consolidated financial statements from the date of acquisition.