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		6 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023
Basic EPS:
Net income (loss) attributable to Cabot Corporation	$ 84	$ 75	$ 134	$ 129
Less: Dividends and dividend equivalents to participating securities		1		1
Less: Undistributed earnings allocated to participating securities	1	1	2	2
Earnings (loss) allocated to common stockholders (numerator)	$ 83	$ 73	$ 132	$ 126
Weighted average common shares and participating securities outstanding	56.2	57.6	56.2	57.6
Less: Participating securities	0.8	1.3	0.9	1.3
Adjusted weighted average common shares (denominator)	55.4	56.3	55.3	56.3
Earnings (loss) per common share - basic:	$ 1.5	$ 1.31	$ 2.39	$ 2.24
Diluted EPS:
Earnings (loss) allocated to common stockholders	$ 83	$ 73	$ 132	$ 126
Plus: Earnings allocated to participating securities	1	2	2	3
Less: Adjusted earnings allocated to participating securities	1	2	2	3
Earnings (loss) allocated to common stockholders (numerator)	$ 83	$ 73	$ 132	$ 126
Adjusted weighted average common shares outstanding	55.4	56.3	55.3	56.3
Common shares issuable	0.4	0.5	0.5	0.4
Adjusted weighted average common shares (denominator)	55.8	56.8	55.8	56.7
Earnings (loss) per common share - diluted:	$ 1.49	$ 1.29	$ 2.37	$ 2.23