Consolidated Statements of Comprehensive Income (Loss) (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 113	$ 120	$ 322	$ 278
Other comprehensive income (loss), net of tax
Foreign currency translation adjustment, net of tax	76	(66)	6	(19)
Derivatives: net investment hedges
(Gains) losses reclassified to interest expense, net of tax	(1)	(1)	(3)	(3)
(Gains) losses excluded from effectiveness testing and amortized to interest expense, net of tax	1	1	2	2
Pension and other post-retirement benefit liability adjustments, net of tax				(1)
Other comprehensive income (loss), net of tax (provision) benefit of $4, $(1), $3, $11	76	(66)	5	(21)
Comprehensive income (loss)	189	54	327	257
Net income (loss) attributable to noncontrolling interests, net of tax	12	11	34	35
Foreign currency translation adjustment attributable to noncontrolling interests, net of tax	4	(1)		(1)
Comprehensive income (loss) attributable to noncontrolling interests	16	10	34	34
Comprehensive income (loss) attributable to Cabot Corporation	$ 173	$ 44	$ 293	$ 223