Earnings Per Share (Components Of Basic And Diluted Earnings Per Common Share) (Details) (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
Basic EPS:
Net income attributable to Cabot Corporation	$ 240		$ 51		$ 286		$ 126
Less: Dividends and dividend equivalents to participating securities			1				1
Less: Undistributed earnings allocated to participating securities	3	[1]	1	[1]	3	[1]	2	[1]
Earnings allocated to common shareholders (numerator)	237		49		283		123
Weighted average common shares and participating securities outstanding	63.8		65.4		64.0		65.4
Less: Participating securities	0.6	[1]	0.8	[1]	0.6	[1]	0.9	[1]
Adjusted weighted average common shares (denominator)	63.2		64.6		63.4		64.5
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 EPS:
Earnings allocated to common shareholders	237		49		283		123
Plus: Earnings allocated to participating securities	3		2		3		3
Earnings allocated to common shareholders (numerator)	237		49		283		123
Less: Adjusted earnings allocated to participating securities	$ (3)	[2]	$ (2)	[2]	$ (3)	[2]	$ (3)	[2]
Adjusted weighted average common shares outstanding	63.2		64.6		63.4		64.5
Common shares issuable	0.8	[3]	0.9	[3]	0.7	[3]	0.8	[3]
Adjusted weighted average shares (denominator)	64.0		65.5		64.1		65.3
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

[1] Participating securities consist of shares of unvested restricted stock, vested restricted stock awards held by employees in which Cabot has a security interest, and unvested time-based restricted stock units.

[2] Undistributed earnings are adjusted for the assumed distribution of dividends to the dilutive securities, which are described in (3) below, and then reallocated to participating securities.

[3] Represents incremental shares of common stock from the (i) assumed exercise of stock options issued under Cabot's equity incentive plans; (ii) assumed issuance of shares to employees pursuant to the Company's Supplemental Retirement Savings Plan; and (iii) assumed issuance of shares under outstanding performance-based stock unit awards issued under Cabot's equity incentive plans. For the three and six months ended March 31, 2012 , 304,000 and 539,000 incremental shares of common stock, respectively, were not included in the calculation of diluted earnings per share because the inclusion of these shares would have been antidilutive. For the three and six months ended March 31, 2011, 198,000 and 273,000 incremental shares of common stock, respectively, were not included in the calculation of diluted earnings per share because the inclusion of these shares would have been antidilutive.