Consolidated Statement Of Changes In Stockholders' Equity And Comprehensive Income (Loss) (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			75					75	75
Foreign currency translation adjustment, net of tax					8			8
Total other comprehensive income								8
Comprehensive income attributable to Cabot Corporation						83		83
Net income attributable to noncontrolling interests, net of tax							5	5	(5)
Comprehensive income attributable to noncontrolling interests								5
Comprehensive income								88	88
Noncontrolling interest-dividends							(1)		(1)
Cash dividends paid to common stockholders			(12)			(12)			(12)
Issuance of stock under employee compensation plans, net of forfeitures, shares	62
Issuance of stock under employee compensation plans, net of forfeitures		2				2			2
Amortization of share-based compensation		4				4			4
Purchase and retirement of common and treasury stock, shares	(23)
Principal payment by Employee Stock Ownership Plan under guaranteed loan				2		2			2
Balance, value at Dec. 31, 2010	63	52	1,188	(18)	96	1,381	119		1,500
Balance, shares at Dec. 31, 2010	65,409
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			46					46	46
Foreign currency translation adjustment, net of tax					(13)			(13)
Change in employee benefit plans, net of tax					(1)			(1)
Total other comprehensive income								(14)
Comprehensive income attributable to Cabot Corporation						32		32
Net income attributable to noncontrolling interests, net of tax							5	5	(5)
Noncontrolling interests-foreign currency translation adjustment							(2)	(2)
Comprehensive income attributable to noncontrolling interests								3
Comprehensive income								35	35
Contribution from noncontrolling interests							4		4
Cash dividends paid to common stockholders			(12)			(12)			(12)
Issuance of stock under employee compensation plans, net of forfeitures, shares	87
Issuance of stock under employee compensation plans, net of forfeitures		2				2			2
Amortization of share-based compensation		4				4			4
Purchase and retirement of common and treasury stock, shares	(690)
Purchase and retirement of common and treasury stock	(1)	(21)				(22)			(22)
Purchase of treasury stock, shares	(250)
Purchase of treasury stock	(8)					(8)			(8)
Principal payment by Employee Stock Ownership Plan under guaranteed loan				2		2			2
Notes receivable for restricted stock-payments		1				1			1
Balance, value at Dec. 31, 2011	$ 54	$ 4	$ 1,348	$ (12)	$ 92	$ 1,486	$ 136		$ 1,622
Balance, shares at Dec. 31, 2011	63,008