Earnings Per Share - Components of Basic and Diluted Earnings (Loss) Per Common Share (EPS) (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Basic EPS:
Net income (loss) attributable to Cabot Corporation	$ 23	$ (173)	$ 92	$ (295)
Earnings (loss) allocated to common shareholders (numerator)	$ 23	$ (173)	$ 92	$ (295)
Weighted average common shares and participating securities outstanding	60.0	62.6	60.4	62.5
Less: Participating securities	0.9	0.8	0.9	0.7
Adjusted weighted average common shares (denominator)	59.1	61.8	59.5	61.8
Earnings (loss) per common share - basic:	$ 0.39	$ (2.80)	$ 1.53	$ (4.78)
Diluted EPS:
Earnings (loss) allocated to common shareholders	$ 23	$ (173)	$ 92	$ (295)
Plus: Earnings (loss) allocated to participating securities			1
Less: Adjusted earnings allocated to participating securities			1
Earnings (loss) allocated to common shareholders (numerator)	$ 23	$ (173)	$ 92	$ (295)
Adjusted weighted average common shares outstanding	59.1	61.8	59.5	61.8
Common shares issuable	0.2		0.2
Adjusted weighted average common shares (denominator)	59.3	61.8	59.7	61.8
Earnings (loss) per common share - diluted:	$ 0.39	$ (2.80)	$ 1.53	$ (4.78)