Earnings Per Share - Components of Basic and Diluted Earnings Per Common Share (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2011
Basic EPS:
Net income attributable to Cabot Corporation	$ 47	$ 59	$ 27	$ 20	$ 36	$ 66	$ 240	$ 46	$ 153		$ 388		$ 236
Less: Dividends and dividend equivalents to participating securities													1
Less: Undistributed earnings allocated to participating securities									1	[1]	3	[1]	2	[1]
Earnings allocated to common shareholders (numerator)									152		385		233
Weighted average common shares and participating securities outstanding									64.4		64.0		65.4
Less: Participating securities									0.6	[1]	0.6	[1]	0.8	[1]
Adjusted weighted average common shares (denominator)									63.8		63.4		64.6
Income from continuing operations attributable to Cabot Corporation	$ 0.72	$ 0.86	$ 0.43	$ 0.33	$ 0.55	$ 0.97	$ 0.80	$ 0.55	$ 2.34		$ 2.87		$ 2.80
Income from discontinued operations	$ 0.02	$ 0.04		$ (0.02)	$ 0.02	$ 0.07	$ 2.94	$ 0.17	$ 0.04		$ 3.20		$ 0.82
Net income attributable to Cabot Corporation	$ 0.74	$ 0.90	$ 0.43	$ 0.31	$ 0.57	$ 1.04	$ 3.74	$ 0.72	$ 2.38		$ 6.07		$ 3.62
Diluted EPS:
Earnings allocated to common shareholders									152		385		233
Plus: Earnings allocated to participating securities									1		3		3
Less: Adjusted earnings allocated to participating securities									(1)	[2]	(3)	[2]	(3)	[2]
Earnings available to common shares (numerator)									$ 152		$ 385		$ 233
Adjusted weighted average common shares outstanding									63.8		63.4		64.6
Common shares issuable									0.7	[3]	0.8	[3]	0.8	[3]
Adjusted weighted average common shares (denominator)									64.5		64.2		65.4
Income from continuing operations attributable to Cabot Corporation	$ 0.71	$ 0.86	$ 0.42	$ 0.33	$ 0.54	$ 0.96	$ 0.78	$ 0.55	$ 2.32		$ 2.83		$ 2.77
Income from discontinued operations	$ 0.02	$ 0.04		$ (0.02)	$ 0.02	$ 0.06	$ 2.92	$ 0.16	$ 0.04		$ 3.16		$ 0.80
Net income attributable to Cabot Corporation	$ 0.73	$ 0.90	$ 0.42	$ 0.31	$ 0.56	$ 1.02	$ 3.70	$ 0.71	$ 2.36		$ 5.99		$ 3.57

[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 for outstanding and achieved performance-based stock unit awards issued under Cabot's equity incentive plans using the treasury stock method. For fiscal 2013, 2012 and 2011, 301,328, 395,532 and 253,000 incremental shares of common stock were not included in the calculation of diluted earnings per share because the inclusion of these shares would have been antidilutive.