Consolidated Statement Of Changes In Stockholders' Equity And Comprehensive Income (Parenthetical) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2012		Mar. 31, 2011
Consolidated Statement Of Changes In Stockholders' Equity And Comprehensive Income [Abstract]
Comprehensive income	$ 264	$ 71	$ 299	[1]	$ 159	[2]
Comprehensive income net of tax attributable to noncontrolling interests	8	8
Comprehensive income attributable to Cabot Corporation, net of tax	$ 256	$ 63

[1] Comprehensive income for the three months ended March 31, 2012 was $264 million, which consists of comprehensive income attributable to Cabot Corporation, net of tax, of $256 million and comprehensive income attributable to noncontrolling interests, net of tax, of $8 million.

[2] Comprehensive income for the three months ended March 31, 2011 was $71 million, which consists of comprehensive income attributable to Cabot Corporation, net of tax, of $63 million and comprehensive income attributable to noncontrolling interests, net of tax, of $8 million.