Earnings Per Share - Calculation of Undistributed Earnings (Detail) - USD ($) $ in Millions	3 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share [Abstract]
Net income (loss) attributable to Cabot Corporation	$ (122)	$ 55
Less: Dividends declared on common stock	20	19
Undistributed earnings (loss)	(142)	36
Undistributed earnings (loss) allocated to common shareholders	(142)	36
Undistributed earnings (loss)	$ (142)	$ 36

Earnings (loss) which remain after dividends declared during the period are assumed to be distributed to participating shareholders.

No definition available.

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Amount of paid and unpaid common stock dividends declared with the form of settlement in cash, stock and payment-in-kind (PIK).

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- Definition

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

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- Definition

The earnings that is allocated to common stock and participating securities to the extent that each security may share in earnings as if all of the earnings for the period had been distributed.

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