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, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Basic EPS:
Net income (loss) attributable to Cabot Corporation	$ 68	$ 45	$ 74	$ 54	$ 52	$ 56	$ 48	$ (7)	$ 241	$ 149	$ (334)
Less: Undistributed earnings allocated to participating securities									2	1
Earnings (loss) allocated to common shareholders (numerator)									$ 239	$ 148	$ (334)
Weighted average common shares and participating securities outstanding									62.8	62.9	63.9
Less: Participating securities									0.5	0.5	0.5
Adjusted weighted average common shares (denominator)									62.3	62.4	63.4
Income (loss) from continuing operations attributable to Cabot Corporation					$ 0.81	$ 0.90	$ 0.76	$ (0.11)	$ 3.83	$ 2.36	$ (5.29)
Income (loss) from discontinued operations					0.02					0.02	0.02
Net income (loss) attributable to Cabot Corporation	$ 1.08	$ 0.71	$ 1.19	$ 0.85	0.83	0.90	0.76	(0.11)	$ 3.83	$ 2.38	$ (5.27)
Diluted EPS:
Earnings (loss) allocated to common shareholders									$ 239	$ 148	$ (334)
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)									$ 239	$ 148	$ (334)
Adjusted weighted average common shares (denominator)									62.3	62.4	63.4
Common shares issuable									0.4	0.5
Adjusted weighted average common shares (denominator)									62.7	62.9	63.4
Income (loss) from continuing operations attributable to Cabot Corporation					0.81	0.88	0.76	(0.11)	$ 3.80	$ 2.34	$ (5.29)
Income (loss) from discontinued operations					0.02					0.02	0.02
Net income (loss) attributable to Cabot Corporation	$ 1.06	$ 0.71	$ 1.18	$ 0.85	$ 0.83	$ 0.88	$ 0.76	$ (0.11)	$ 3.80	$ 2.36	$ (5.27)