Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ 29	$ (163)	$ 106	$ (275)
Other comprehensive income (loss), net of tax
Foreign currency translation adjustment, net of tax (provision) benefit of $(1), $2, $(3) and $4	8	59	(16)	59
Derivatives: net investment hedges
(Gains) losses reclassified to interest expense, net of tax provision (benefit) of $1, $—, $1 and $—	(1)	(1)	(2)	(2)
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)	7	58	4	57
Comprehensive income (loss)	36	(105)	110	(218)
Net income (loss) attributable to noncontrolling interests, net of tax	6	10	14	20
Foreign currency translation adjustment attributable to noncontrolling interests, net of tax	2	4	2	7
Comprehensive income (loss) attributable to noncontrolling interests, net of tax	8	14	16	27
Comprehensive income (loss) attributable to Cabot Corporation	$ 28	$ (119)	$ 94	$ (245)