Earnings Per Share - Calculation of Undistributed Earnings (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Earnings Per Share [Abstract]
Net income (loss) attributable to Cabot Corporation	$ 101	$ 109	$ 288	$ 243
Less: Dividends declared on common stock	24	24	70	69
Less: Dividends declared on participating securities	0	0	1	0
Undistributed earnings (loss)	77	85	217	174
Undistributed earnings (loss) allocated to common shareholders	76	84	214	171
Undistributed earnings allocated to participating shareholders	$ 1	$ 1	$ 3	$ 3