Consolidated Statements of Comprehensive (Loss) Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Accumulated Other Comprehensive Income Loss Net Of Tax [Abstract]
Net (loss) income	$ (443)	$ 57	$ (367)	$ 182
Other comprehensive income (loss), net of tax
Foreign currency translation adjustment (net of tax provision of $-, $-, $1 and $1)	18	11	(216)	(18)
Unrealized holding gains arising during the period (net of tax provision of $-, $1, $- and $-)		2		2
Pension and other postretirement benefit liability adjustments
Pension and other postretirement benefit liability adjustments arising during the period (net of tax provision of $-, $-, $6 and $-)			21
Amortization of net loss and prior service credit included in net periodic pension cost (net of tax benefit of less than $1 million in all periods)			1	1
Other comprehensive income (loss)	18	13	(194)	(15)
Comprehensive (loss) income	(425)	70	(561)	167
Net income attributable to noncontrolling interests	2	5	7	14
Noncontrolling interests foreign currency translation adjustment, net of tax			(3)	(2)
Comprehensive income attributable to noncontrolling interests, net of tax	2	5	4	12
Comprehensive (loss) income attributable to Cabot Corporation	$ (427)	$ 65	$ (565)	$ 155