Earnings Per Share - Calculation of Undistributed Earnings (Detail) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Earnings Per Share [Abstract]
Net income (loss) attributable to Cabot Corporation	$ 60	$ 41
Less: Dividends declared on common stock	20	20
Undistributed earnings (loss)	40	21
Undistributed earnings (loss) allocated to common shareholders	$ 40	$ 21