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		9 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Basic EPS:
Net (loss) income attributable to Cabot Corporation	$ (445)	$ 52	$ (374)	$ 168
Less: Undistributed earnings allocated to participating securities		1		2
(Loss) earnings allocated to common shareholders (numerator)	$ (445)	$ 51	$ (374)	$ 166
Weighted average common shares and participating securities outstanding	63.8	65.1	64.2	64.9
Less: Participating securities	0.5	0.6	0.5	0.6
Adjusted weighted average common shares (denominator)	63.3	64.5	63.7	64.3
(Loss) income from continuing operations attributable to Cabot Corporation	$ (7.05)	$ 0.80	$ (5.89)	$ 2.61
Income (loss) from discontinued operations	0.01	(0.01)	0.01	(0.03)
Net (loss) income attributable to Cabot Corporation	$ (7.04)	$ 0.79	$ (5.88)	$ 2.58
Diluted EPS(2):
(Loss) earnings allocated to common shareholders	$ (445)	$ 51	$ (374)	$ 166
Plus: (Loss) earnings allocated to participating securities		1		2
Less: Adjusted earnings allocated to participating securities		(1)		(2)
(Loss) earnings allocated to common shareholders (numerator)	$ (445)	$ 51	$ (374)	$ 166
Adjusted weighted average common shares outstanding	63.3	64.5	63.7	64.3
Common shares issuable		0.7		0.7
Adjusted weighted average common shares (denominator)	63.3	65.2	63.7	65.0
(Loss) income from continuing operations attributable to Cabot Corporation	$ (7.05)	$ 0.79	$ (5.89)	$ 2.58
Income (loss) from discontinued operations	0.01	(0.01)	0.01	(0.03)
Net (loss) income attributable to Cabot Corporation	$ (7.04)	$ 0.78	$ (5.88)	$ 2.55