Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ (74)	$ 273	$ 162
Other comprehensive income (loss), net of tax
Foreign currency translation adjustment, net of tax (provision) benefit of $1, $4, and $—	(64)	25	7
Unrealized holding gains (losses) arising during the period, net of tax provision of $—, $—, and $—	(1)
Derivatives: net investment hedges
(Gains) losses reclassified to interest expense, net of tax provision (benefit) of $2, $—, and $—	(3)
(Gains) losses excluded from effectiveness testing and amortized to interest expense, net of tax provision (benefit) of $(1), $—, and $—	1
Pension and other postretirement benefit liability adjustments
Pension and other postretirement benefit liability adjustments arising during the period, net of tax	6	41	(38)
Amortization of net loss and prior service credit included in net periodic pension cost, net of tax	(1)	2
Other comprehensive income (loss)	(62)	68	(31)
Comprehensive income (loss)	(136)	341	131
Net income (loss) attributable to noncontrolling interests, net of tax	39	25	15
Foreign currency translation adjustment attributable to noncontrolling interests, net of tax	(4)	2	(5)
Comprehensive income (loss) attributable to noncontrolling interests	35	27	10
Comprehensive income (loss) attributable to Cabot Corporation	$ (171)	$ 314	$ 121