Earnings Per Share - Components of Basic and Diluted Earnings (Loss) Per Common Share (EPS) (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Basic EPS:
Net income (loss) attributable to Cabot Corporation	$ 88	$ 47	$ (207)	$ 176
Less: Undistributed earnings allocated to participating securities				1
Earnings (loss) allocated to common shareholders (numerator)	$ 88	$ 47	$ (207)	$ 175
Weighted average common shares and participating securities outstanding	62.5	62.9	62.5	62.8
Less: Participating securities	0.7	0.5	0.7	0.5
Adjusted weighted average common shares (denominator)	61.8	62.4	61.8	62.3
Earnings (loss) per common share - basic:	$ 1.41	$ 0.73	$ (3.36)	$ 2.79
Diluted EPS:
Earnings (loss) allocated to common shareholders	$ 88	$ 47	$ (207)	$ 175
Plus: Earnings (loss) allocated to participating securities	1		1	1
Less: Adjusted earnings allocated to participating securities	1		1	1
Earnings (loss) allocated to common shareholders (numerator)	$ 88	$ 47	$ (207)	$ 175
Adjusted weighted average common shares outstanding	61.8	62.4	61.8	62.3
Common shares issuable	0.5	0.3		0.5
Adjusted weighted average common shares (denominator)	62.3	62.7	61.8	62.8
Earnings (loss) per common share - diluted:	$ 1.40	$ 0.73	$ (3.36)	$ 2.78