Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ 53	$ 60	$ 191	$ 109
Other comprehensive income (loss), net of tax
Foreign currency translation adjustment, net of tax provision (benefit) of $(4), $—, $(1) and $—	54	(13)	(15)	5
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 benefit cost, net of tax		(1)	2
Other comprehensive income (loss)	54	(14)	(13)	4
Comprehensive income (loss)	107	46	178	113
Net income (loss) attributable to noncontrolling interests, net of tax	8	4	18	12
Foreign currency translation adjustment attributable to noncontrolling interests, net of tax	3	(3)		(5)
Comprehensive income (loss) attributable to noncontrolling interests, net of tax	11	1	18	7
Comprehensive income (loss) attributable to Cabot Corporation	$ 96	$ 45	$ 160	$ 106