Earnings Per Share - Calculation of Undistributed Earnings (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017	Mar. 31, 2016
Earnings Per Share [Abstract]
Net income (loss) attributable to Cabot Corporation	$ 74	$ 48	$ 128	$ 41
Less: Dividends declared on common stock	19	14	38	28
Undistributed earnings (loss)	55	34	90	13
Undistributed earnings (loss) allocated to common shareholders	54	$ 34	89	$ 13
Undistributed earnings (loss) allocated to participating shareholders	$ 1		$ 1