Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ (112)	$ 59
Other comprehensive income (loss), net of tax
Foreign currency translation adjustment, net of tax provision (benefit) of $(2) and $2		(125)
Derivatives: net investment hedges
(Gains) losses reclassified to interest expense, net of tax provision of $— and $—	(1)
Pension and other postretirement benefit liability adjustments
Amortization of net loss and prior service credit included in net periodic benefit cost, net of tax		1
Other comprehensive income (loss)	(1)	(124)
Comprehensive income (loss)	(113)	(65)
Net income (loss) attributable to noncontrolling interests, net of tax	10	4
Foreign currency translation adjustment attributable to noncontrolling interests, net of tax	3	(4)
Comprehensive income (loss) attributable to noncontrolling interests, net of tax	13
Comprehensive income (loss) attributable to Cabot Corporation	$ (126)	$ (65)