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, 2020	Dec. 31, 2019
Basic EPS:
Net income (loss) attributable to Cabot Corporation	$ 60	$ 41
Earnings (loss) allocated to common shareholders (numerator)	$ 60	$ 41
Weighted average common shares and participating securities outstanding	57.2	57.7
Less: Participating securities	0.7	0.8
Adjusted weighted average common shares (denominator)	56.5	56.9
Earnings (loss) per common share - basic:	$ 1.06	$ 0.71
Diluted EPS:
Earnings (loss) allocated to common shareholders	$ 60	$ 41
Earnings (loss) allocated to common shareholders (numerator)	$ 60	$ 41
Adjusted weighted average common shares outstanding	56.5	56.9
Common shares issuable	0.1	0.1
Adjusted weighted average common shares (denominator)	56.6	57.0
Earnings (loss) per common share - diluted:	$ 1.06	$ 0.70