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, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Basic EPS:
Net income (loss) attributable to Cabot Corporation	$ 109	$ 82	$ 243	$ 211
Less: Dividends and dividend equivalents to participating securities				1
Less: Undistributed earnings allocated to participating securities	1	1	3	3
Earnings (loss) allocated to common stockholders (numerator)	$ 108	$ 81	$ 240	$ 207
Weighted average common shares and participating securities outstanding	55.9	57.3	56.2	57.5
Less: Participating securities	0.8	1.2	0.9	1.3
Adjusted weighted average common shares (denominator)	55.1	56.1	55.3	56.2
Earnings (loss) per common share - basic:	$ 1.96	$ 1.44	$ 4.34	$ 3.68
Diluted EPS:
Earnings (loss) allocated to common stockholders	$ 108	$ 81	$ 240	$ 207
Plus: Earnings allocated to participating securities	1	1	3	4
Less: Adjusted earnings allocated to participating securities	1	1	3	4
Earnings (loss) allocated to common stockholders (numerator)	$ 108	$ 81	$ 240	$ 207
Adjusted weighted average common shares outstanding	55.1	56.1	55.3	56.2
Common shares issuable	0.6	0.4	0.5	0.5
Adjusted weighted average common shares (denominator)	55.7	56.5	55.8	56.7
Earnings (loss) per common share - diluted:	$ 1.94	$ 1.43	$ 4.3	$ 3.65