Consolidated Statements of Comprehensive Income (Loss) (Unaudited) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Mar. 31, 2025	Mar. 31, 2024
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 105	$ 97	$ 209	$ 158
Other comprehensive income (loss), net of tax
Foreign currency translation adjustment, net of tax	34	(15)	(70)	47
Derivatives: net investment hedges
(Gains) losses reclassified to interest expense, net of tax	(1)	(1)	(2)	(2)
(Gains) losses excluded from effectiveness testing and amortized to interest expense, net of tax		1	1	1
Pension and other post-retirement benefit liability adjustments, net of tax		(1)		(1)
Other comprehensive income (loss), net of tax (provision) benefit of $3, $11, $(1), $12	33	(16)	(71)	45
Comprehensive income (loss)	138	81	138	203
Net income (loss) attributable to noncontrolling interests, net of tax	11	13	22	24
Foreign currency translation adjustment attributable to noncontrolling interests, net of tax	3	(4)	(4)
Comprehensive income (loss) attributable to noncontrolling interests	14	9	18	24
Comprehensive income (loss) attributable to Cabot Corporation	$ 124	$ 72	$ 120	$ 179