Earnings Per Share - Calculation of Undistributed Earnings (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Earnings Per Share [Abstract]
Net income attributable to Cabot Corporation	$ 52	$ 59	$ 168	$ 106
Less: Dividends declared on common stock	14	13	40	39
Less: Dividends declared on participating securities
Undistributed earnings	38	46	128	67
Undistributed earnings allocated to common shareholders	37	45	126	66
Undistributed earnings allocated to participating shareholders	$ 1	$ 1	$ 2	$ 1