Earnings Per Share - Calculation of Undistributed Earnings (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Mar. 31, 2025	Mar. 31, 2024
Earnings Per Share [Abstract]
Net income (loss) attributable to Cabot Corporation	$ 94	$ 84	$ 187	$ 134
Less: Dividends declared on common stock	22	23	46	45
Less: Dividends declared on participating securities	1		1
Undistributed earnings (loss)	71	61	140	89
Undistributed earnings (loss) allocated to common shareholders	70	60	138	87
Undistributed earnings allocated to participating shareholders	$ 1	$ 1	$ 2	$ 2