Consolidated Statements Of Comprehensive (Loss) Income (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013	Mar. 31, 2012
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 23	$ 245	$ 47	$ 296
Other comprehensive (loss) income, net of tax
Foreign currency translation adjustment	(36)	19	(22)	5
Adjustments to pension and other postretirement benefit plans	2	(1)	3	(2)
Net change in unrealized gain on investments	1		1
Other comprehensive (loss) income	(33)	18	(18)	3
Comprehensive (loss) income	(10)	263	29	299
Net (loss) income attributable to noncontrolling interests	(4)	5		10
Noncontrolling interests foreign currency translation adjustment	(1)	2		1
Comprehensive (loss) income attributable to noncontrolling interests	(5)	7		11
Comprehensive (loss) income attributable to Cabot Corporation	$ (5)	$ 256	$ 29	$ 288