Earnings Per Share - Calculation of Undistributed Earnings (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Earnings Per Share [Abstract]
Net income (loss) attributable to Cabot Corporation	$ 68	$ 45	$ 74	$ 54	$ 52	$ 56	$ 48	$ (7)	$ 241	$ 149	$ (334)
Less: Dividends declared on common stock									77	65	56
Undistributed earnings (loss)									164	84	(390)
Undistributed earnings (loss) allocated to common shareholders									162	83	$ (390)
Undistributed earnings allocated to participating securities									$ 2	$ 1