Earnings Per Share - Components of Basic and Diluted Earnings Per Common Share (EPS) (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017	Mar. 31, 2016
Basic EPS:
Net income (loss) attributable to Cabot Corporation	$ 74	$ 48	$ 128	$ 41
Less: Undistributed earnings allocated to participating securities	1		1
Earnings (loss) allocated to common shareholders (numerator)	$ 73	$ 48	$ 127	$ 41
Weighted average common shares and participating securities outstanding	62.9	62.9	62.8	62.9
Less: Participating securities	0.5	0.5	0.5	0.5
Adjusted weighted average common shares (denominator)	62.4	62.4	62.3	62.4
Earnings per common share - basic:	$ 1.19	$ 0.76	$ 2.04	$ 0.65
Diluted EPS:
Earnings (loss) allocated to common shareholders	$ 73	$ 48	$ 127	$ 41
Plus: Earnings (loss) allocated to participating securities	1		1
Less: Adjusted earnings allocated to participating securities	1		1
Earnings (loss) allocated to common shareholders (numerator)	$ 73	$ 48	$ 127	$ 41
Adjusted weighted average common shares (denominator)	62.4	62.4	62.3	62.4
Common shares issuable	0.4	0.4	0.5	0.5
Adjusted weighted average common shares (denominator)	62.8	62.8	62.8	62.9
Earnings per common share - diluted:	$ 1.18	$ 0.76	$ 2.03	$ 0.65