Consolidated Statements of Comprehensive Income (Loss) (Unaudited) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ 85	$ 3	$ 155	$ 49
Other comprehensive income (loss), net of tax
Foreign currency translation adjustment, net of tax	(76)	(78)	18	(35)
Derivatives: net investment hedges
(Gains) losses reclassified to interest expense, net of tax	(2)	(1)	(3)	(2)
(Gains) losses excluded from effectiveness testing and amortized to interest expense, net of tax	1	1	1	1
Pension and other postretirement benefit liability adjustments, net of tax	1		4	1
Other comprehensive income (loss)	(76)	(78)	20	(35)
Comprehensive income (loss)	9	(75)	175	14
Net income (loss) attributable to noncontrolling interests, net of tax	10	4	20	9
Foreign currency translation adjustment attributable to noncontrolling interests, net of tax	(2)	(3)	5
Comprehensive income (loss) attributable to noncontrolling interests, net of tax	8	1	25	9
Comprehensive income (loss) attributable to Cabot Corporation	$ 1	$ (76)	$ 150	$ 5