Earnings Per Share - Calculation of Undistributed Earnings (Detail) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Earnings Per Share [Abstract]
Net (loss) income attributable to Cabot Corporation	$ (7)	$ 45
Less: Dividends declared on common stock	14	14
Undistributed (loss) earnings	(21)	31
Undistributed (loss) earnings allocated to common shareholders	$ (21)	$ 31