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, 2024	Dec. 31, 2023
Basic EPS:
Net income (loss) attributable to Cabot Corporation	$ 93	$ 50
Less: Undistributed earnings allocated to participating securities	1	1
Earnings (loss) allocated to common stockholders (numerator)	$ 92	$ 49
Weighted average common shares and participating securities outstanding	55.2	56.4
Less: Participating securities	0.9	1.1
Adjusted weighted average common shares (denominator)	54.3	55.3
Earnings (loss) per common share - basic:	$ 1.69	$ 0.88
Diluted EPS:
Earnings (loss) allocated to common stockholders	$ 92	$ 49
Plus: Earnings allocated to participating securities	1	1
Less: Adjusted earnings allocated to participating securities	1	1
Earnings (loss) allocated to common stockholders (numerator)	$ 92	$ 49
Adjusted weighted average common shares outstanding	54.3	55.3
Common shares issuable	0.7	0.5
Adjusted weighted average common shares (denominator)	55.0	55.8
Earnings (loss) per common share - diluted:	$ 1.67	$ 0.88