Consolidated Statements of Comprehensive Income (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Amounts Reclassified Out Of Accumulated Other Comprehensive Income Loss [Abstract]
Net income	$ 62	$ 70	$ 109	$ 366
Other comprehensive (loss) income, net of tax
Foreign currency translation adjustment	(16)	(40)	(38)	(35)
Adjustments to pension and other postretirement benefit plans	13	(1)	16	(3)
Net change in unrealized gain on investments	1		1
Other comprehensive loss	(2)	(41)	(21)	(38)
Comprehensive income	60	29	88	328
Net income attributable to noncontrolling interests	3	4	3	14
Noncontrolling interests foreign currency translation adjustment	2	(4)	1	(3)
Comprehensive income attributable to noncontrolling interests	5		4	11
Comprehensive income attributable to Cabot Corporation	$ 55	$ 29	$ 84	$ 317