Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ 77	$ (112)
Other comprehensive income (loss), net of tax
Foreign currency translation adjustment, net of tax provision (benefit) of $2 and $(2)	(24)
Derivatives: net investment hedges
(Gains) losses reclassified to interest expense, net of tax provision (benefit) of $— and $—	(1)	(1)
Pension and other postretirement benefit liability adjustments
Pension and other postretirement benefit liability adjustments arising during the period, net of tax	22
Other comprehensive income (loss)	(3)	(1)
Comprehensive income (loss)	74	(113)
Net income (loss) attributable to noncontrolling interests, net of tax	8	10
Foreign currency translation adjustment attributable to noncontrolling interests, net of tax		3
Comprehensive income (loss) attributable to noncontrolling interests, net of tax	8	13
Comprehensive income (loss) attributable to Cabot Corporation	$ 66	$ (126)