Consolidated Statements of Comprehensive Income (USD $) In Millions, unless otherwise specified	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Net income	$ 218	$ 160	$ 406
Other comprehensive (loss) income, net of tax
Foreign currency translation adjustment (net of tax (benefit) provision of $(10), $(12), $2)	(131)	(10)	3
Unrealized holding gains (losses) arising during the period (net of tax provision of $-, $1, $-)	0	2	(1)
Pension and other postretirement benefit liability adjustments
Pension and other postretirement benefit liability adjustments arising during the period (net of tax (benefit) provision of $(1), $13, $(9))	(40)	20	(18)
Amortization of net loss and prior service credit included in net periodic pension cost (net of tax provision of $-, $-, $-)	0	2	1
Other comprehensive (loss) income	(171)	14	(15)
Comprehensive income	47	174	391
Net income attributable to noncontrolling interests, (net of tax provision of $5, $5, $7)	19	7	18
Noncontrolling interests foreign currency translation adjustment	(4)	3	(1)
Comprehensive income attributable to noncontrolling interests	15	10	17
Comprehensive income attributable to Cabot Corporation	$ 32	$ 164	$ 374