Earnings Per Share - Calculation of Undistributed Earnings (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Mar. 31, 2023	Mar. 31, 2022
Earnings Per Share [Abstract]
Net income (loss) attributable to Cabot Corporation	$ 75	$ 107	$ 129	$ 18
Less: Dividends declared on common stock	21	21	42	42
Less: Dividends declared on participating securities	1	1	1	1
Undistributed earnings (loss)	53	85	86	(25)
Undistributed earnings (loss) allocated to common shareholders	52	84	84	$ (25)
Undistributed earnings allocated to participating shareholders	$ 1	$ 1	$ 2