Consolidated Statements of Comprehensive Income (Loss) (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ 95	$ (5)	$ 250	$ 44
Other comprehensive income (loss), net of tax
Foreign currency translation adjustment, net of tax	71	31	89	(4)
Derivatives: net investment hedges
(Gains) losses reclassified to interest expense, net of tax	(1)	(1)	(4)	(3)
(Gains) losses excluded from effectiveness testing and amortized to interest expense, net of tax			1	1
Pension and other postretirement benefit liability adjustments, net of tax	2	1	6	2
Other comprehensive income (loss)	72	31	92	(4)
Comprehensive income (loss)	167	26	342	40
Net income (loss) attributable to noncontrolling interests, net of tax	9	1	29	10
Foreign currency translation adjustment attributable to noncontrolling interests, net of tax	3	2	8	2
Comprehensive income (loss) attributable to noncontrolling interests, net of tax	12	3	37	12
Comprehensive income (loss) attributable to Cabot Corporation	$ 155	$ 23	$ 305	$ 28