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Earnings Per Share - Calculation of Undistributed Earnings (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Earnings Per Share [Abstract]
Net income (loss) attributable to Cabot Corporation	$ 331	$ 380	$ 445
Less: Dividends declared on common stock	95	92	87
Less: Dividends and dividend equivalents to participating securities	1	1	1
Undistributed earnings (loss)	235	287	357
Undistributed earnings (loss) allocated to common shareholders	231	282	350
Undistributed earnings allocated to participating securities	$ 4	$ 5	$ 7

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