Earnings Per Share - Calculation of Undistributed Earnings (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Earnings Per Share [Abstract]
Net income (loss) attributable to Cabot Corporation	$ (173)	$ 74	$ (295)	$ 129
Less: Dividends declared on common stock	19	19	39	38
Undistributed earnings (loss)	(192)	55	(334)	91
Undistributed earnings (loss) allocated to common shareholders	$ (192)	54	$ (334)	90
Undistributed earnings (loss) allocated to participating shareholders		$ 1		$ 1