Earnings Per Share - Calculation of Undistributed Earnings (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2022	Mar. 31, 2021
Earnings Per Share [Abstract]
Net income (loss) attributable to Cabot Corporation	$ 107	$ 75	$ 18	$ 135
Less: Dividends declared on common stock	21	20	42	40
Less: Dividends declared on participating securities	1		1
Undistributed earnings (loss)	85	55	(25)	95
Undistributed earnings (loss) allocated to common shareholders	84	54	$ (25)	94
Undistributed earnings allocated to participating shareholders	$ 1	$ 1		$ 1