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, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Basic EPS:
Net income (loss) attributable to Cabot Corporation	$ 94	$ 88	$ (173)	$ (122)	$ 72	$ 47	$ 74	$ 55	$ (113)	$ 248	$ 147
Less: Dividends and dividend equivalents to participating securities									1
Less: Undistributed earnings allocated to participating securities										2	1
Earnings (loss) allocated to common shareholders (numerator)									$ (114)	$ 246	$ 146
Weighted average common shares and participating securities outstanding									62.4	62.8	62.9
Less: Participating securities									0.7	0.5	0.5
Adjusted weighted average common shares (denominator)									61.7	62.3	62.4
Income (loss) from continuing operations attributable to Cabot Corporation									$ (1.85)	$ 3.94	$ 2.32
Income (loss) from discontinued operations											0.02
Net income (loss) attributable to Cabot Corporation	$ 1.51	$ 1.41	$ (2.80)	$ (1.98)	$ 1.15	$ 0.73	$ 1.19	$ 0.87	$ (1.85)	$ 3.94	$ 2.34
Diluted EPS:
Earnings (loss) allocated to common shareholders									$ (114)	$ 246	$ 146
Plus: Earnings (loss) allocated to participating securities										2	1
Less: Adjusted earnings allocated to participating securities										2	1
Earnings (loss) available to common shares (numerator)									$ (114)	$ 246	$ 146
Adjusted weighted average common shares outstanding									61.7	62.3	62.4
Common shares issuable										0.4	0.5
Adjusted weighted average common shares (denominator)									61.7	62.7	62.9
Income (loss) from continuing operations attributable to Cabot Corporation									$ (1.85)	$ 3.91	$ 2.30
Income (loss) from discontinued operations											0.02
Net income (loss) attributable to Cabot Corporation	$ 1.51	$ 1.40	$ (2.80)	$ (1.98)	$ 1.13	$ 0.73	$ 1.19	$ 0.86	$ (1.85)	$ 3.91	$ 2.32