Earnings Per Share - Calculation of Undistributed Earnings (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023
Earnings Per Share [Abstract]
Net income (loss) attributable to Cabot Corporation	$ 84	$ 75	$ 134	$ 129
Less: Dividends declared on common stock	23	21	45	42
Less: Dividends declared on participating securities		1		1
Undistributed earnings (loss)	61	53	89	86
Undistributed earnings (loss) allocated to common shareholders	60	52	87	84
Undistributed earnings allocated to participating shareholders	$ 1	$ 1	$ 2	$ 2