Earnings Per Share - Calculation of Undistributed Earnings (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Earnings Per Share [Abstract]
Net income (loss) attributable to Cabot Corporation	$ (272)	$ (6)	$ (1)	$ 41	$ 33	$ 32	$ 23	$ 69	$ (238)	$ 157	$ (113)
Less: Dividends declared on common stock									80	80	79
Less: Dividends and dividend equivalents to participating securities										1	1
Undistributed earnings (loss)									(318)	76	(193)
Undistributed earnings (loss) allocated to common shareholders									$ (318)	75	$ (193)
Undistributed earnings allocated to participating securities										$ 1