Earnings Per Share (Calculation of Undistributed Earnings) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2010
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net income attributable to Cabot Corporation	$ 36	$ 66	$ 240	$ 46	$ 50	$ 60	$ 51	$ 75	$ 388	$ 236	$ 154
Less: Dividends declared on common stock									49	46	46
Less: Dividends and dividend equivalents to participating securities										1	1
Undistributed earnings									339	189	107
Undistributed earnings allocated to common shareholders									336	187	105
Undistributed earnings allocated to participating securities									$ 3	$ 2	$ 2