Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ 266	$ 164	$ (326)
Other comprehensive income (loss), net of tax
Foreign currency translation adjustment, net of tax (provision) benefit of $4, $—, $3	25	7	(270)
Pension and other postretirement benefit liability adjustments
Pension and other postretirement benefit liability adjustments arising during the period, net of tax	41	(38)	28
Amortization of net loss and prior service credit included in net periodic pension cost, net of tax	2		3
Other comprehensive income (loss)	68	(31)	(239)
Comprehensive income (loss)	334	133	(565)
Net income (loss) attributable to noncontrolling interests, net of tax	25	15	8
Foreign currency translation adjustment attributable to noncontrolling interests, net of tax	2	(5)	(4)
Comprehensive income (loss) attributable to noncontrolling interests	27	10	4
Comprehensive income (loss) attributable to Cabot Corporation	$ 307	$ 123	$ (569)