Consolidated Statements of Comprehensive (Loss) Income (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Statement Of Income And Comprehensive Income [Abstract]
Net (loss) income	$ (3)	$ 49
Other comprehensive loss, net of tax
Foreign currency translation adjustment (net of tax benefit of $- and $1)	(47)	(103)
Pension and other postretirement benefit liability adjustments
Pension and other postretirement benefit liability adjustments arising during the period (net of tax provision of $- and $6)	(1)	21
Other comprehensive loss	(48)	(82)
Comprehensive loss	(51)	(33)
Net income attributable to noncontrolling interests	4	4
Noncontrolling interests foreign currency translation adjustment, net of tax	(3)	(2)
Comprehensive income attributable to noncontrolling interests, net of tax	1	2
Comprehensive loss attributable to Cabot Corporation	$ (52)	$ (35)