Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ 58	$ (3)
Other comprehensive income (loss), net of tax
Foreign currency translation adjustment (net of tax provision of $2 and $—)	(125)	(47)
Pension and other postretirement benefit liability adjustments
Pension and other postretirement benefit liability adjustments arising during the period, net of tax		(1)
Amortization of net loss and prior service credit included in net periodic pension cost, net of tax	1
Other comprehensive income (loss)	(124)	(48)
Comprehensive income (loss)	(66)	(51)
Net income (loss) attributable to noncontrolling interests, net of tax	4	4
Noncontrolling interests foreign currency translation adjustment, net of tax	(4)	(3)
Comprehensive income (loss) attributable to noncontrolling interests, net of tax		1
Comprehensive income (loss) attributable to Cabot Corporation	$ (66)	$ (52)