Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ 164	$ (326)	$ 218
Other comprehensive income (loss), net of tax
Foreign currency translation adjustment, net of tax benefit of $—, $(3), ($10)	7	(270)	(131)
Pension and other postretirement benefit liability adjustments
Pension and other postretirement benefit liability adjustments arising during the period, net of tax	(38)	28	(40)
Amortization of net loss and prior service credit included in net periodic pension cost, net of tax		3
Other comprehensive loss	(31)	(239)	(171)
Comprehensive income (loss)	133	(565)	47
Net income attributable to noncontrolling interests, net of tax	15	8	19
Noncontrolling interests foreign currency translation adjustment	(5)	(4)	(4)
Comprehensive income attributable to noncontrolling interests	10	4	15
Comprehensive income (loss) attributable to Cabot Corporation	$ 123	$ (569)	$ 32