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
	Dec. 31, 2018	Dec. 31, 2017
Basic EPS:
Net income (loss) attributable to Cabot Corporation	$ 69	$ (122)
Earnings (loss) allocated to common shareholders (numerator)	$ 69	$ (122)
Weighted average common shares and participating securities outstanding	60.8	62.6
Less: Participating securities	0.9	0.7
Adjusted weighted average common shares (denominator)	59.9	61.9
Earnings (loss) per common share - basic:	$ 1.14	$ (1.98)
Diluted EPS:
Earnings (loss) allocated to common shareholders	$ 69	$ (122)
Plus: Earnings (loss) allocated to participating securities	1
Less: Adjusted earnings allocated to participating securities	1
Earnings (loss) allocated to common shareholders (numerator)	$ 69	$ (122)
Adjusted weighted average common shares outstanding	59.9	61.9
Common shares issuable	0.2
Adjusted weighted average common shares (denominator)	60.1	61.9
Earnings (loss) per common share - diluted:	$ 1.14	$ (1.98)