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
	Dec. 31, 2023	Dec. 31, 2022
Basic EPS:
Net income (loss) attributable to Cabot Corporation	$ 50	$ 54
Less: Undistributed earnings allocated to participating securities	1	1
Earnings (loss) allocated to common stockholders (numerator)	$ 49	$ 53
Weighted average common shares and participating securities outstanding	56.4	57.6
Less: Participating securities	1.1	1.3
Adjusted weighted average common shares (denominator)	55.3	56.3
Earnings (loss) per common share - basic:	$ 0.88	$ 0.94
Diluted EPS:
Earnings (loss) allocated to common stockholders	$ 49	$ 53
Plus: Earnings allocated to participating securities	1	1
Less: Adjusted earnings allocated to participating securities	1	1
Earnings (loss) allocated to common stockholders (numerator)	$ 49	$ 53
Adjusted weighted average common shares outstanding	55.3	56.3
Common shares issuable	0.5	0.4
Adjusted weighted average common shares (denominator)	55.8	56.7
Earnings (loss) per common share - diluted:	$ 0.88	$ 0.93