Earnings Per Share - Calculation of Undistributed Earnings (Detail) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Earnings Per Share [Abstract]
Net income (loss) attributable to Cabot Corporation	$ 50	$ 54
Less: Dividends declared on common stock	22	21
Undistributed earnings (loss)	28	33
Undistributed earnings (loss) allocated to common shareholders	27	32
Undistributed earnings allocated to participating shareholders	$ 1	$ 1