Earnings Per Share - Components of Basic and Diluted Earnings Per Common Share (EPS) (Detail) - USD ($) $ / shares in Units, shares in Millions, $ 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
Basic EPS:
Net income (loss) attributable to Cabot Corporation	$ (272)	$ (6)	$ (1)	$ 41	$ 33	$ 32	$ 23	$ 69	$ (238)	$ 157	$ (113)
Less: Dividends and dividend equivalents to participating securities										1	1
Less: Undistributed earnings allocated to participating securities										1
Earnings (loss) allocated to common shareholders (numerator)									$ (238)	$ 155	$ (114)
Weighted average common shares and participating securities outstanding									57.3	59.5	62.4
Less: Participating securities									0.7	0.8	0.7
Adjusted weighted average common shares (denominator)									56.6	58.7	61.7
Basic	$ (4.81)	$ (0.12)	$ (0.01)	$ 0.71	$ 0.56	$ 0.55	$ 0.39	$ 1.14	$ (4.21)	$ 2.64	$ (1.85)
Diluted EPS:
Earnings (loss) allocated to common shareholders									$ (238)	$ 155	$ (114)
Plus: Earnings allocated to participating securities										2
Less: Adjusted earnings allocated to participating securities										2
Earnings (loss) available to common shares (numerator)									$ (238)	$ 155	$ (114)
Adjusted weighted average common shares outstanding									56.6	58.7	61.7
Common shares issuable										0.1
Adjusted weighted average common shares (denominator)									56.6	58.8	61.7
Diluted	$ (4.81)	$ (0.12)	$ (0.01)	$ 0.70	$ 0.55	$ 0.55	$ 0.39	$ 1.14	$ (4.21)	$ 2.63	$ (1.85)