Income Tax - Effective Tax Rate (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Income Tax Disclosure [Abstract]
(Provision) benefit for income taxes	$ (43)	$ (40)	$ (133)	$ (121)
Effective tax rate	28.00%	25.00%	29.00%	31.00%

Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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+ Details

CABOT CORP 10-Q