Earnings Per Share - Calculation of Undistributed Earnings (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Earnings Per Share [Abstract]
Net income (loss) attributable to Cabot Corporation	$ 97	$ 86	$ 115	$ 221
Less: Dividends declared on common stock	21	20	63	60
Less: Dividends declared on participating securities		1	1	1
Undistributed earnings (loss)	76	65	51	160
Undistributed earnings (loss) allocated to common shareholders	75	64	50	158
Undistributed earnings allocated to participating shareholders	$ 1	$ 1	$ 1	$ 2