Earnings Per Share - Components of Basic and Diluted Earnings Per Common Share (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Basic EPS:
Net income attributable to Cabot Corporation	$ 52	$ 59	$ 168	$ 106
Less: Dividends and dividend equivalents to participating securities
Less: Undistributed earnings allocated to participating securities	1	1	2	1
Earnings allocated to common shareholders (numerator)	51	58	166	105
Weighted average common shares and participating securities outstanding	65.1	64.4	64.9	64.2
Less: Participating securities	0.6	0.6	0.6	0.6
Adjusted weighted average common shares (denominator)	64.5	63.8	64.3	63.6
Income from continuing operations attributable to Cabot Corporation	$ 0.80	$ 0.88	$ 2.61	$ 1.66
(Loss) income from discontinued operations	$ (0.01)	$ 0.04	$ (0.03)	$ (0.01)
Net income attributable to Cabot Corporation	$ 0.79	$ 0.92	$ 2.58	$ 1.65
Diluted EPS:
Earnings allocated to common shareholders	51	58	166	105
Plus: Earnings allocated to participating securities	1	1	2	1
Less: Adjusted earnings allocated to participating securities	(1)	(1)	(2)	(1)
Earnings allocated to common shareholders (numerator)	$ 51	$ 58	$ 166	$ 105
Adjusted weighted average common shares outstanding	64.5	63.8	64.3	63.6
Common shares issuable	0.7	0.7	0.7	0.7
Adjusted weighted average common shares (denominator)	65.2	64.5	65.0	64.3
Income from continuing operations attributable to Cabot Corporation	$ 0.79	$ 0.87	$ 2.58	$ 1.64
(Loss) income from discontinued operations	$ (0.01)	$ 0.03	$ (0.03)	$ (0.01)
Net income attributable to Cabot Corporation	$ 0.78	$ 0.90	$ 2.55	$ 1.63