Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ 99	$ 55	$ (176)	$ 194
Other comprehensive income (loss), net of tax
Foreign currency translation adjustment, net of tax provision (benefit) of $2, $(4), $(2) and $(1)	(109)	54	(50)	(15)
Unrealized holding gains (losses) arising during the period, net of tax provision (benefit) of $—, $—, $— and $—	(1)		(1)
Derivatives: net investment hedges
(Gains) losses reclassified to interest expense, net of tax provision of $1, $—, $1 and $—	(1)		(3)
Pension and other postretirement benefit liability adjustments
Amortization of net loss and prior service credit included in net periodic benefit cost, net of tax				2
Other comprehensive income (loss)	(111)	54	(54)	(13)
Comprehensive income (loss)	(12)	109	(230)	181
Net income (loss) attributable to noncontrolling interests, net of tax	11	8	31	18
Foreign currency translation adjustment attributable to noncontrolling interests, net of tax	(7)	3
Comprehensive income (loss) attributable to noncontrolling interests, net of tax	4	11	31	18
Comprehensive income (loss) attributable to Cabot Corporation	$ (16)	$ 98	$ (261)	$ 163