Unaudited Quarterly Financial Information - Schedule of Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [Abstract]
Net sales and other operating revenues	$ 659	$ 518	$ 710	$ 727	$ 827	$ 845	$ 844	$ 821	$ 2,614	$ 3,337	$ 3,242
Gross profit	137	69	153	141	171	170	178	166	500	685	772
Net income (loss)	(265)	(5)	3	46	40	40	29	77	(221)	186	(74)
Net income (loss) attributable to Cabot Corporation	$ (272)	$ (6)	$ (1)	$ 41	$ 33	$ 32	$ 23	$ 69	$ (238)	$ 157	$ (113)
Earnings per common share—basic	$ (4.81)	$ (0.12)	$ (0.01)	$ 0.71	$ 0.56	$ 0.55	$ 0.39	$ 1.14	$ (4.21)	$ 2.64	$ (1.85)
Earnings per common share—diluted	$ (4.81)	$ (0.12)	$ (0.01)	$ 0.70	$ 0.55	$ 0.55	$ 0.39	$ 1.14	$ (4.21)	$ 2.63	$ (1.85)