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, 2023	Sep. 30, 2022	Sep. 30, 2021
Basic EPS:
Net income (loss) attributable to Cabot Corporation	$ 445	$ 209	$ 250
Less: Dividends and dividend equivalents to participating securities	1	1	1
Less: Undistributed earnings allocated to participating securities	7	2	2
Earnings (loss) allocated to common shareholders (numerator)	$ 437	$ 206	$ 247
Weighted average common shares and participating securities outstanding	57.3	57.4	57.5
Less: Participating securities	1.2	0.9	0.8
Adjusted weighted average common shares (denominator)	56.1	56.5	56.7
Basic	$ 7.79	$ 3.65	$ 4.35
Diluted EPS:
Earnings (loss) allocated to common shareholders	$ 437	$ 206	$ 247
Plus: Earnings allocated to participating securities	8	3	3
Less: Adjusted earnings allocated to participating securities	8	3	3
Earnings (loss) available to common shares (numerator)	$ 437	$ 206	$ 247
Adjusted weighted average common shares outstanding	56.1	56.5	56.7
Common shares issuable	0.4	0.4	0.1
Adjusted weighted average common shares (denominator)	56.5	56.9	56.8
Diluted	$ 7.73	$ 3.62	$ 4.34