Earnings Per Share - Calculation of Undistributed Earnings (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Earnings Per Share [Abstract]
Net income (loss) attributable to Cabot Corporation	$ 82	$ 97	$ 211	$ 115
Less: Dividends declared on common stock	23	21	64	63
Less: Dividends declared on participating securities			1	1
Undistributed earnings (loss)	59	76	146	51
Undistributed earnings (loss) allocated to common shareholders	58	75	143	50
Undistributed earnings allocated to participating shareholders	$ 1	$ 1	$ 3	$ 1