Earnings Per Share - Calculation of Undistributed Earnings (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Earnings Per Share [Abstract]
Net income (loss) attributable to Cabot Corporation	$ 250	$ (238)	$ 157
Less: Dividends declared on common stock	80	80	80
Less: Dividends and dividend equivalents to participating securities	1		1
Undistributed earnings (loss)	169	(318)	76
Undistributed earnings (loss) allocated to common shareholders	167	$ (318)	75
Undistributed earnings allocated to participating securities	$ 2		$ 1