Earnings Per Share - Calculation of Undistributed Earnings (Detail) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Earnings Per Share [Abstract]
Net income (loss) attributable to Cabot Corporation	$ (89)	$ 60
Less: Dividends declared on common stock	21	20
Undistributed earnings (loss)	(110)	40
Undistributed earnings (loss) allocated to common shareholders	$ (110)	$ 40