Earnings Per Share (Calculation Of Undistributed Earnings) (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2012	Mar. 31, 2011
Earnings Per Share [Abstract]
Net income attributable to Cabot Corporation	$ 240	$ 51	$ 286	$ 126
Less: Dividends declared on common stock	12	11	24	23
Less: Dividends declared on participating securities		1		1
Undistributed earnings allocated to common shareholders	225	38	259	100
Undistributed earnings allocated to participating shareholders	3	1	3	2
Undistributed earnings	$ 228	$ 39	$ 262	$ 102
Shares excluded from calculation of EPS	304,000	198,000	539,000	273,000