Earnings Per Share - Calculation of Undistributed Earnings (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Earnings Per Share [Abstract]
Net income (loss) attributable to Cabot Corporation	$ 56	$ (445)	$ 97	$ (374)
Less: Dividends declared on common stock	19	14	47	42
Undistributed earnings (loss)	37	(459)	50	(416)
Undistributed earnings (loss) allocated to common shareholders	$ 37	$ (459)	$ 50	$ (416)