Consolidated Statements of Comprehensive Income (Loss) (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ (80)	$ 70
Other comprehensive income (loss), net of tax
Foreign currency translation adjustment, net of tax	(28)	94
Derivatives: net investment hedges
(Gains) losses reclassified to interest expense, net of tax	(1)	(1)
Pension and other postretirement benefit liability adjustments, net of tax	1	3
Other comprehensive income (loss), net of tax of $— and $(3)	(28)	96
Comprehensive income (loss)	(108)	166
Net income (loss) attributable to noncontrolling interests, net of tax	9	10
Foreign currency translation adjustment attributable to noncontrolling interests, net of tax	2	7
Comprehensive income (loss) attributable to noncontrolling interests	11	17
Comprehensive income (loss) attributable to Cabot Corporation	$ (119)	$ 149