Earnings Per Share - Components of Basic and Diluted Earnings Per Common Share (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Basic EPS:
Net (loss) income attributable to Cabot Corporation	$ (7)	$ 45
(Loss) earnings allocated to common shareholders (numerator)	$ (7)	$ 45
Weighted average common shares and participating securities outstanding	62.9	64.6
Less: Participating securities	0.4	0.5
Adjusted weighted average common shares (denominator)	62.5	64.1
Net (loss) income attributable to Cabot Corporation	$ (0.11)	$ 0.70
Diluted EPS:
(Loss) earnings allocated to common shareholders	$ (7)	$ 45
(Loss) earnings allocated to common shareholders (numerator)	$ (7)	$ 45
Adjusted weighted average common shares (denominator)	62.5	64.1
Common shares issuable		0.5
Adjusted weighted average common shares (denominator)	62.5	64.6
Net (loss) income attributable to Cabot Corporation	$ (0.11)	$ 0.69