Earnings Per Share - Components of Basic and Diluted Earnings (Loss) Per Common Share (EPS) (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Basic EPS:
Net income (loss) attributable to Cabot Corporation	$ 82	$ 97	$ 211	$ 115
Less: Dividends and dividend equivalents to participating securities			1	1
Less: Undistributed earnings allocated to participating securities	1	1	3	1
Earnings (loss) allocated to common shareholders (numerator)	$ 81	$ 96	$ 207	$ 113
Weighted average common shares and participating securities outstanding	57.3	57.2	57.5	57.4
Less: Participating securities	1.2	0.9	1.3	0.9
Adjusted weighted average common shares (denominator)	56.1	56.3	56.2	56.5
Earnings (loss) per common share - basic:	$ 1.44	$ 1.70	$ 3.68	$ 2.00
Diluted EPS:
Earnings (loss) allocated to common shareholders	$ 81	$ 96	$ 207	$ 113
Plus: Earnings allocated to participating securities	1	1	4	2
Less: Adjusted earnings allocated to participating securities	1	1	4	2
Earnings (loss) allocated to common shareholders (numerator)	$ 81	$ 96	$ 207	$ 113
Adjusted weighted average common shares outstanding	56.1	56.3	56.2	56.5
Common shares issuable	0.4	0.5	0.5	0.4
Adjusted weighted average common shares (denominator)	56.5	56.8	56.7	56.9
Earnings (loss) per common share - diluted:	$ 1.43	$ 1.69	$ 3.65	$ 1.99