Earnings Per Share - Calculation of Undistributed Earnings (Detail) - USD ($) $ in Millions	3 Months Ended
Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net income (loss) attributable to Cabot Corporation	$ 54	$ (7)
Less: Dividends declared on common stock	19	14
Undistributed earnings (loss)	35	(21)
Undistributed earnings (loss) allocated to common shareholders	$ 35	$ (21)

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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- Definition

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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