Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 484	$ 243	$ 286
Other comprehensive income (loss), net of tax
Foreign currency translation adjustment, net of tax	80	(175)	52
Derivatives: net investment hedges
(Gains) losses reclassified to interest expense, net of tax	(6)	(6)	(5)
(Gains) losses excluded from effectiveness testing and amortized to interest expense, net of tax	2	2	2
Pension and other postretirement benefit liability adjustments, net of tax	1	14	20
Other comprehensive income (loss), net of tax of $1, $3 and $8	77	(165)	69
Comprehensive income (loss)	561	78	355
Net Income (Loss) Attributable to Noncontrolling Interest	39	34	36
Foreign currency translation adjustment attributable to noncontrolling interests, net of tax		(15)	7
Comprehensive income (loss) attributable to noncontrolling interests	39	19	43
Comprehensive income (loss) attributable to Cabot Corporation	$ 522	$ 59	$ 312