Earnings Per Share - Components of Basic and Diluted Earnings Per Common Share (EPS) (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Basic EPS:
Net income (loss) attributable to Cabot Corporation	$ 250	$ (238)	$ 157
Less: Dividends and dividend equivalents to participating securities	1		1
Less: Undistributed earnings allocated to participating securities	2		1
Earnings (loss) allocated to common shareholders (numerator)	$ 247	$ (238)	$ 155
Weighted average common shares and participating securities outstanding	57.5	57.3	59.5
Less: Participating securities	0.8	0.7	0.8
Adjusted weighted average common shares (denominator)	56.7	56.6	58.7
Basic	$ 4.35	$ (4.21)	$ 2.64
Diluted EPS:
Earnings (loss) allocated to common shareholders	$ 247	$ (238)	$ 155
Plus: Earnings allocated to participating securities	3		2
Less: Adjusted earnings allocated to participating securities	3		2
Earnings (loss) available to common shares (numerator)	$ 247	$ (238)	$ 155
Adjusted weighted average common shares outstanding	56.7	56.6	58.7
Common shares issuable	0.1		0.1
Adjusted weighted average common shares (denominator)	56.8	56.6	58.8
Diluted	$ 4.34	$ (4.21)	$ 2.63