Earnings Per Share - Calculation of Undistributed Earnings (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Earnings Per Share [Abstract]
Net income (loss) attributable to Cabot Corporation	$ 86	$ (6)	$ 221	$ 34
Less: Dividends declared on common stock	20	20	60	60
Less: Dividends declared on participating securities	1		1
Undistributed earnings (loss)	65	(26)	160	(26)
Undistributed earnings (loss) allocated to common shareholders	64	$ (26)	158	$ (26)
Undistributed earnings allocated to participating shareholders	$ 1		$ 2