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
	Dec. 31, 2014	Dec. 31, 2013
Basic EPS:
Net income attributable to Cabot Corporation	$ 45	$ 80
Less: Dividends and dividend equivalents to participating securities	0	0
Less: Undistributed earnings allocated to participating securities		1
Earnings allocated to common shareholders (numerator)	45	79
Weighted average common shares and participating securities outstanding	64.6	64.8
Less: Participating securities	0.5	0.6
Adjusted weighted average common shares (denominator)	64.1	64.2
Income from continuing operations attributable to Cabot Corporation	$ 0.70	$ 1.25
Loss from discontinued operations		$ (0.01)
Net income attributable to Cabot Corporation	$ 0.70	$ 1.24
Diluted EPS:
Earnings allocated to common shareholders	45	79
Plus: Earnings allocated to participating securities		1
Less: Adjusted earnings allocated to participating securities		(1)
Earnings allocated to common shareholders (numerator)	$ 45	$ 79
Adjusted weighted average common shares outstanding	64.1	64.2
Common shares issuable	0.5	0.6
Adjusted weighted average common shares (denominator)	64.6	64.8
Income from continuing operations attributable to Cabot Corporation	$ 0.69	$ 1.24
Loss from discontinued operations		$ (0.01)
Net income attributable to Cabot Corporation	$ 0.69	$ 1.23