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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Basic EPS:
Net income (loss) attributable to Cabot Corporation	$ 32	$ 88	$ 124	$ (207)
Earnings (loss) allocated to common shareholders (numerator)	$ 32	$ 88	$ 124	$ (207)
Weighted average common shares and participating securities outstanding	59.0	62.5	59.9	62.5
Less: Participating securities	0.8	0.7	0.8	0.7
Adjusted weighted average common shares (denominator)	58.2	61.8	59.1	61.8
Earnings (loss) per common share - basic:	$ 0.55	$ 1.41	$ 2.08	$ (3.36)
Diluted EPS:
Earnings (loss) allocated to common shareholders	$ 32	$ 88	$ 124	$ (207)
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)	$ 32	$ 88	$ 124	$ (207)
Adjusted weighted average common shares outstanding	58.2	61.8	59.1	61.8
Common shares issuable	0.2	0.5	0.1
Adjusted weighted average common shares (denominator)	58.4	62.3	59.2	61.8
Earnings (loss) per common share - diluted:	$ 0.55	$ 1.40	$ 2.08	$ (3.36)