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								12 Months Ended
	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Basic EPS:
Net (loss) income attributable to Cabot Corporation	$ 40	$ (445)	$ 26	$ 45	$ 31	$ 52	$ 36	$ 80	$ (334)	$ 199	$ 153
Less: Dividends and dividend equivalents to participating securities										1
Less: Undistributed earnings allocated to participating securities										1	1
(Loss) earnings allocated to common shareholders (numerator)									$ (334)	$ 197	$ 152
Weighted average common shares and participating securities outstanding									63.9	65.0	64.4
Less: Participating securities									0.5	0.6	0.6
Adjusted weighted average common shares (denominator)									63.4	64.4	63.8
(Loss) income from continuing operations attributable to Cabot Corporation	$ 0.63	$ (7.05)	$ 0.41	$ 0.70	$ 0.43	$ 0.80	$ 0.56	$ 1.25	$ (5.29)	$ 3.04	$ 2.39
Income (loss) from discontinued operations	0.01	0.01			0.05	(0.01)	(0.01)	(0.01)	0.02	0.02	(0.01)
Net (loss) income attributable to Cabot Corporation	0.64	(7.04)	0.41	0.70	0.48	0.79	0.55	1.24	$ (5.27)	$ 3.06	$ 2.38
Diluted EPS:
(Loss) earnings allocated to common shareholders									$ (334)	$ 197	$ 152
Plus: Earnings allocated to participating securities										1	1
Less: Adjusted earnings allocated to participating securities										1	1
(Loss) earnings available to common shares (numerator)									$ (334)	$ 197	$ 152
Adjusted weighted average common shares (denominator)									63.4	64.4	63.8
Common shares issuable										0.7	0.7
Adjusted weighted average common shares(denominator)									63.4	65.1	64.5
(Loss) income from continuing operations attributable to Cabot Corporation	0.62	(7.05)	0.41	0.69	0.43	0.79	0.55	1.24	$ (5.29)	$ 3.01	$ 2.37
Income (loss) from discontinued operations	0.01	0.01			0.05	(0.01)	(0.01)	(0.01)	0.02	0.02	(0.01)
Net (loss) income attributable to Cabot Corporation	$ 0.63	$ (7.04)	$ 0.41	$ 0.69	$ 0.48	$ 0.78	$ 0.54	$ 1.23	$ (5.27)	$ 3.03	$ 2.36