Consolidated Statements Of Operations (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended				6 Months Ended
	Mar. 31, 2012		Mar. 31, 2011		Mar. 31, 2012		Mar. 31, 2011
Consolidated Statements Of Operations [Abstract]
Net sales and other operating revenues	$ 844	[1]	$ 739	[1]	$ 1,606	[1]	$ 1,433	[1]
Cost of sales	671		605		1,290		1,168
Gross profit	173		134		316		265
Selling and administrative expenses	66		62		131		125
Research and technical expenses	20		18		37		33
Income from operations	87		54		148		107
Interest and dividend income	1				2		1
Interest expense	(9)		(10)		(19)		(20)
Other (expense) income	(3)		4				6
Income from continuing operations before income taxes and equity in net earnings of affiliated companies	76	[2]	48	[2]	131	[2]	94	[2]
(Provision) benefit for income taxes	(23)		(9)		(39)		6
Equity in net earnings of affiliated companies	3		1		4		4
Income from continuing operations	56		40		96		104
Income from discontinued operations, net of tax	189		16		200		32
Net income	245		56		296		136
Net income attributable to noncontrolling interests, net of tax	5		5		10		10
Net income attributable to Cabot Corporation	$ 240		$ 51		$ 286		$ 126
Weighted-average common shares outstanding, in millions:
Basic	63.2		64.6		63.4		64.5
Diluted	64.0		65.5		64.1		65.3
Basic:
Income from continuing operations attributable to Cabot Corporation	$ 0.80		$ 0.53		$ 1.35		$ 1.42
Income from discontinued operations	$ 2.94		$ 0.24		$ 3.11		$ 0.49
Net income attributable to Cabot Corporation	$ 3.74		$ 0.77		$ 4.46		$ 1.91
Diluted:
Income from continuing operations attributable to Cabot Corporation	$ 0.78		$ 0.52		$ 1.33		$ 1.40
Income from discontinued operations	$ 2.92		$ 0.24		$ 3.08		$ 0.49
Net income attributable to Cabot Corporation	$ 3.70		$ 0.76		$ 4.41		$ 1.89
Dividends per common share	$ 0.18		$ 0.18		$ 0.36		$ 0.36

[1] Unallocated and Other reflects royalties paid by equity affiliates, external shipping and handling fees, and other operating revenues, which includes the impact of the corporate adjustment for unearned revenue.

[2] Unallocated and Other includes certain items and eliminations that are not allocated to the operating segments. Management does not consider these items necessary for an understanding of the operating results of these segments and such amounts are excluded in the segment reporting to the Chief Operating Decision Maker. Income (loss) from continuing operations before taxes that are categorized as Unallocated and Other includes: