Consolidated Statements of Comprehensive Income (Loss) (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 104	$ 61
Other comprehensive income (loss), net of tax
Foreign currency translation adjustment, net of tax	(104)	62
Derivatives: net investment hedges
(Gains) losses reclassified to interest expense, net of tax	(1)	(1)
(Gains) losses excluded from effectiveness testing and amortized to interest expense, net of tax	1
Other comprehensive income (loss), net of tax (provision) benefit of $(4), $1	(104)	61
Comprehensive income (loss)		122
Net income (loss) attributable to noncontrolling interests, net of tax	11	11
Foreign currency translation adjustment attributable to noncontrolling interests, net of tax	(7)	4
Comprehensive income (loss) attributable to noncontrolling interests	4	15
Comprehensive income (loss) attributable to Cabot Corporation	$ (4)	$ 107