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, 2025	Mar. 31, 2024	Mar. 31, 2025	Mar. 31, 2024
Basic EPS:
Net income (loss) attributable to Cabot Corporation	$ 94	$ 84	$ 187	$ 134
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)	$ 92	$ 83	$ 184	$ 132
Weighted average common shares and participating securities outstanding	54.8	56.2	55.0	56.2
Less: Participating securities	0.8	0.8	0.8	0.9
Adjusted weighted average common shares (denominator)	54.0	55.4	54.2	55.3
Earnings (loss) per common share - basic:	$ 1.71	$ 1.5	$ 3.4	$ 2.39
Diluted EPS:
Earnings (loss) allocated to common stockholders	$ 92	$ 83	$ 184	$ 132
Plus: Earnings allocated to participating securities	2	1	3	2
Less: Adjusted earnings allocated to participating securities	2	1	3	2
Earnings (loss) allocated to common stockholders (numerator)	$ 92	$ 83	$ 184	$ 132
Adjusted weighted average common shares outstanding	54.0	55.4	54.2	55.3
Common shares issuable	0.4	0.4	0.5	0.5
Adjusted weighted average common shares (denominator)	54.4	55.8	54.7	55.8
Earnings (loss) per common share - diluted:	$ 1.69	$ 1.49	$ 3.36	$ 2.37