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, 2021	Dec. 31, 2020
Basic EPS:
Net income (loss) attributable to Cabot Corporation	$ (89)	$ 60
Earnings (loss) allocated to common shareholders (numerator)	$ (89)	$ 60
Weighted average common shares and participating securities outstanding	57.7	57.2
Less: Participating securities	0.9	0.7
Adjusted weighted average common shares (denominator)	56.8	56.5
Earnings (loss) per common share - basic:	$ (1.57)	$ 1.06
Diluted EPS:
Earnings (loss) allocated to common shareholders	$ (89)	$ 60
Earnings (loss) allocated to common shareholders (numerator)	$ (89)	$ 60
Adjusted weighted average common shares outstanding	56.8	56.5
Common shares issuable		0.1
Adjusted weighted average common shares (denominator)	56.8	56.6
Earnings (loss) per common share - diluted:	$ (1.57)	$ 1.06