Earnings Per Share - Calculation of Undistributed Earnings (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
Earnings Per Share [Abstract]
Net income attributable to Cabot Corporation	$ 48	$ 26	$ 41	$ 71
Less: Dividends declared on common stock	14	14	28	28
Undistributed earnings	34	12	13	43
Undistributed earnings allocated to common shareholders	$ 34	$ 12	$ 13	$ 43