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, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Basic EPS:
Net income (loss) attributable to Cabot Corporation	$ 86	$ (6)	$ 221	$ 34
Less: Dividends and dividend equivalents to participating securities	1		1
Less: Undistributed earnings allocated to participating securities	1		2
Earnings (loss) allocated to common shareholders (numerator)	$ 84	$ (6)	$ 218	$ 34
Weighted average common shares and participating securities outstanding	57.5	57.2	57.4	57.4
Less: Participating securities	0.8	0.7	0.7	0.7
Adjusted weighted average common shares (denominator)	56.7	56.5	56.7	56.7
Earnings (loss) per common share - basic:	$ 1.48	$ (0.12)	$ 3.85	$ 0.59
Diluted EPS:
Earnings (loss) allocated to common shareholders	$ 84	$ (6)	$ 218	$ 34
Plus: Earnings allocated to participating securities	1		2
Less: Adjusted earnings allocated to participating securities	1		2
Earnings (loss) allocated to common shareholders (numerator)	$ 84	$ (6)	$ 218	$ 34
Adjusted weighted average common shares outstanding	56.7	56.5	56.7	56.7
Common shares issuable	0.3		0.1
Adjusted weighted average common shares (denominator)	57.0	56.5	56.8	56.7
Earnings (loss) per common share - diluted:	$ 1.48	$ (0.12)	$ 3.84	$ 0.59