Earnings Per Share - Calculation of Undistributed Earnings (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Earnings Per Share [Abstract]
Net income (loss) attributable to Cabot Corporation	$ 23	$ (173)	$ 92	$ (295)
Less: Dividends declared on common stock	20	19	40	39
Undistributed earnings (loss)	3	(192)	52	(334)
Undistributed earnings (loss) allocated to common shareholders	$ 3	$ (192)	$ 52	$ (334)