Earnings Per Share - Calculation of Undistributed Earnings (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Earnings Per Share [Abstract]
Net income (loss) attributable to Cabot Corporation	$ 75	$ (1)	$ 135	$ 40
Less: Dividends declared on common stock	20	20	40	$ 40
Undistributed earnings (loss)	55	(21)	95
Undistributed earnings (loss) allocated to common shareholders	54	$ (21)	94
Undistributed earnings allocated to participating shareholders	$ 1		$ 1