Consolidated Statement Of Changes In Stockholders' Equity And Comprehensive Income (USD $) In Millions, except Share data in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Deferred Employee Benefits [Member]	Accumulated Other Comprehensive Income [Member]	Total Cabot Corporation Stockholders' Equity [Member]	Non-controlling Interest [Member]	Comprehensive Income [Member]
Beginning Balance at Sep. 30, 2009	$ 1,237	$ 63	$ 18	$ 1,018	$ (25)	$ 60	$ 1,134	$ 103
Beginning Balance, Shares at Sep. 30, 2009		65,309
Net income attributable to Cabot Corporation	154			154					154
Foreign currency translation adjustment, net of tax						43			43
Change in employee benefit plans, net of tax						(15)			(15)
Total other comprehensive income									28
Comprehensive income attributable to Cabot Corporation							182		182
Net income attributable to noncontrolling interests, net of tax	15							15	15
Noncontrolling interests - foreign currency translation adjustment								2	2
Comprehensive income attributable to noncontrolling interests									17
Comprehensive income	199								199
Noncontrolling interest-other	1							1
Noncontrolling interest - dividends	(6)							(6)
Cash dividends paid to common stockholders	(47)			(47)			(47)
Issuance of stock under employee compensation plans, net of forfeitures	9	1	8				9
Issuance of stock under employee compensation plans, net of forfeitures, Shares		283
Amortization of share-based compensation	18		18				18
Purchase and retirement of common and treasury stock	(5)	(1)	(4)				(5)
Purchase and retirement of common and treasury stock, Shares		(222)
Principal payment by Employee Stock Ownership Plan under guaranteed loan	5				5		5
Notes receivable for restricted stock-payments	6		6				6
Ending Balance at Sep. 30, 2010	1,417	63	46	1,125	(20)	88	1,302	115
Ending Balance, Shares at Sep. 30, 2010		65,370
Net income attributable to Cabot Corporation	236			236					236
Foreign currency translation adjustment, net of tax						19			19
Change in unrealized loss on investments and derivative instruments, net of tax						(1)			(1)
Total other comprehensive income									18
Comprehensive income attributable to Cabot Corporation							254		254
Net income attributable to noncontrolling interests, net of tax	22							22	22
Noncontrolling interests - foreign currency translation adjustment								3	3
Comprehensive income attributable to noncontrolling interests									25
Comprehensive income	279								279
Noncontrolling interest - dividends	(11)							(11)
Cash dividends paid to common stockholders	(47)			(47)			(47)
Issuance of stock under employee compensation plans, net of forfeitures	8	1	7				8
Issuance of stock under employee compensation plans, net of forfeitures, Shares		294
Amortization of share-based compensation	17		17				17
Purchase and retirement of common and treasury stock	(59)	(1)	(58)				(59)
Purchase and retirement of common and treasury stock, Shares		(1,803)
Principal payment by Employee Stock Ownership Plan under guaranteed loan	6				6		6
Notes receivable for restricted stock-payments	6		6				6
Ending Balance at Sep. 30, 2011	1,616	63	18	1,314	(14)	106	1,487	129
Ending Balance, Shares at Sep. 30, 2011		63,861
Net income attributable to Cabot Corporation	388			388					388
Foreign currency translation adjustment, net of tax						4			4
Change in unrealized loss on investments and derivative instruments, net of tax						(1)			(1)
Change in employee benefit plans, net of tax						(17)			(17)
Total other comprehensive income									(14)
Comprehensive income attributable to Cabot Corporation							374		374
Net income attributable to noncontrolling interests, net of tax	18							18	18
Noncontrolling interests - foreign currency translation adjustment								(1)	(1)
Comprehensive income attributable to noncontrolling interests									17
Comprehensive income	391								391
Noncontrolling interest-other	4							4
Noncontrolling interest - dividends	(24)							(24)
Cash dividends paid to common stockholders	(49)			(49)			(49)
Issuance of stock under employee compensation plans, net of forfeitures	15	2	13				15
Issuance of stock under employee compensation plans, net of forfeitures, Shares		611
Amortization of share-based compensation	15		15				15
Purchase and retirement of common and treasury stock	(36)	(9)	(27)				(36)
Purchase and retirement of common and treasury stock, Shares		(1,124)
Principal payment by Employee Stock Ownership Plan under guaranteed loan	6				6		6
Notes receivable for restricted stock-payments	1		1				1
Ending Balance at Sep. 30, 2012	$ 1,939	$ 56	$ 20	$ 1,653	$ (8)	$ 92	$ 1,813	$ 126
Ending Balance, Shares at Sep. 30, 2012		63,348