Earnings Per Share - Calculation of Undistributed Earnings (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Earnings Per Share [Abstract]
Net income (loss) attributable to Cabot Corporation	$ 445	$ 209	$ 250
Less: Dividends declared on common stock	87	84	80
Less: Dividends and dividend equivalents to participating securities	1	1	1
Undistributed earnings (loss)	357	124	169
Undistributed earnings (loss) allocated to common shareholders	350	122	167
Undistributed earnings allocated to participating securities	$ 7	$ 2	$ 2