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, 2022	Dec. 31, 2021
Basic EPS:
Net income (loss) attributable to Cabot Corporation	$ 54	$ (89)
Less: Undistributed earnings allocated to participating securities	1
Earnings (loss) allocated to common shareholders (numerator)	$ 53	$ (89)
Weighted average common shares and participating securities outstanding	57.6	57.7
Less: Participating securities	1.3	0.9
Adjusted weighted average common shares (denominator)	56.3	56.8
Earnings (loss) per common share - basic:	$ 0.94	$ (1.57)
Diluted EPS:
Earnings (loss) allocated to common shareholders	$ 53	$ (89)
Plus: Earnings allocated to participating securities	1
Less: Adjusted earnings allocated to participating securities	1
Earnings (loss) allocated to common shareholders (numerator)	$ 53	$ (89)
Adjusted weighted average common shares outstanding	56.3	56.8
Common shares issuable	0.4
Adjusted weighted average common shares (denominator)	56.7	56.8
Earnings (loss) per common share - diluted:	$ 0.93	$ (1.57)