Consolidated Statements of Comprehensive Income (Loss) (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ 70	$ 46
Other comprehensive income (loss), net of tax
Foreign currency translation adjustment, net of tax	94	43
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	3	1
Other comprehensive income (loss)	96	43
Comprehensive income (loss)	166	89
Net income (loss) attributable to noncontrolling interests, net of tax	10	5
Foreign currency translation adjustment attributable to noncontrolling interests, net of tax	7	3
Comprehensive income (loss) attributable to noncontrolling interests, net of tax	17	8
Comprehensive income (loss) attributable to Cabot Corporation	$ 149	$ 81