Earnings Per Share (Calculation of Undistributed Earnings) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Earnings Per Share [Abstract]
Net income attributable to Cabot Corporation	$ 59	$ 66	$ 106	$ 352
Less: Dividends declared on common stock	13	12	39	36
Less: Dividends declared on participating securities
Undistributed earnings	46	54	67	316
Undistributed earnings allocated to common shareholders	45	54	66	313
Undistributed earnings allocated to participating shareholders	$ 1		$ 1	$ 3