Earnings Per Share - Calculation of Undistributed Earnings (Detail) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
Earnings Per Share [Abstract]
Net income (loss) attributable to Cabot Corporation	$ 93	$ 50
Less: Dividends declared on common stock	24	22
Undistributed earnings (loss)	69	28
Undistributed earnings (loss) allocated to common shareholders	68	27
Undistributed earnings allocated to participating shareholders	$ 1	$ 1