Consolidated Statement Of Changes In Stockholders' Equity And Comprehensive Income (USD $) In Millions, except Share data in Thousands		Common Stock, Net Of Treasury Stock [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	Deferred Employee Benefits [Member]	Accumulated Other Comprehensive Income [Member]	Cabot Corporation Stockholders' Equity [Member]	Non-Controlling Interests [Member]	Comprehensive Income [Member]	Total
Balance, value at Sep. 30, 2010		$ 63	$ 46	$ 1,125	$ (20)	$ 88	$ 1,302	$ 115		$ 1,417
Balance, shares at Sep. 30, 2010		65,370
Net income attributable to Cabot Corporation				126					126	126
Foreign currency translation adjustment, net of tax						20			20
Total other comprehensive income (loss)									20
Comprehensive income attributable to Cabot Corporation, net of tax	[1]						146		146
Net income attributable to noncontrolling interests, net of tax								10	10	(10)
Noncontrolling interests foreign currency adjustment								3	3
Comprehensive income attributable to noncontrolling interests, net of tax	[1]								13
Comprehensive income	[1]								159	159
Noncontrolling interests - dividends								(4)		(4)
Cash dividends paid to common stockholders				(24)			(24)			(24)
Issuance of stock under employee compensation plans, net of forfeitures, Shares		218
Issuance of stock under employee compensation plans, net of forfeitures		2	5				7			7
Amortization of share-based compensation			9				9			9
Purchase and retirement of stock, shares		26
Principal payment by Employee Stock Ownership Plan under guaranteed loan					3		3			3
Balance, value at Mar. 31, 2011		65	60	1,227	(17)	108	1,443	124		1,567
Balance, shares at Mar. 31, 2011		65,562
Balance, value at Sep. 30, 2011		63	18	1,314	(14)	106	1,487	129		1,616
Balance, shares at Sep. 30, 2011		63,861
Net income attributable to Cabot Corporation				286					286	286
Foreign currency translation adjustment, net of tax						4			4
Change in employee benefit plans, net of tax						(2)			(2)
Total other comprehensive income (loss)									2
Comprehensive income attributable to Cabot Corporation, net of tax	[2]						288		288
Net income attributable to noncontrolling interests, net of tax								10	10	(10)
Noncontrolling interests foreign currency adjustment								1	1
Comprehensive income attributable to noncontrolling interests, net of tax	[2]								11
Comprehensive income	[2]								299	299
Contribution from noncontrolling interests								4		4
Noncontrolling interests - dividends								(12)		(12)
Cash dividends paid to common stockholders				(24)			(24)			(24)
Issuance of stock under employee compensation plans, net of forfeitures, Shares		484
Issuance of stock under employee compensation plans, net of forfeitures		2	11				13			13
Amortization of share-based compensation			8				8			8
Purchase and retirement of stock, shares		(695)
Purchase and retirement of stock		(1)	(21)				(22)			(22)
Purchase of treasury stock, shares		(250)
Purchase of treasury stock		(8)					(8)			(8)
Notes receivable for restricted stock-payments			1				1			1
Principal payment by Employee Stock Ownership Plan under guaranteed loan					3		3			3
Balance, value at Mar. 31, 2012		$ 56	$ 17	$ 1,576	$ (11)	$ 108	$ 1,746	$ 132		$ 1,878
Balance, shares at Mar. 31, 2012		63,400

[1] 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.

[2] 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.