Unaudited Quarterly Financial Information - Schedule of Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [Abstract]
Net sales and other operating revenues	$ 850	$ 854	$ 818	$ 720	$ 723	$ 705	$ 678	$ 611	$ 3,242	$ 2,717	$ 2,411
Gross profit	213	200	190	178	174	161	169	159	781	663	575
Net income (loss)	102	99	(163)	(112)	79	55	80	59	(74)	273	162
Net income (loss) attributable to Cabot Corporation	$ 94	$ 88	$ (173)	$ (122)	$ 72	$ 47	$ 74	$ 55	$ (113)	$ 248	$ 147
Earnings (loss) per common share - basic:	$ 1.51	$ 1.41	$ (2.80)	$ (1.98)	$ 1.15	$ 0.73	$ 1.19	$ 0.87	$ (1.85)	$ 3.94	$ 2.34
Earnings (loss) per common share - diluted:	$ 1.51	$ 1.40	$ (2.80)	$ (1.98)	$ 1.13	$ 0.73	$ 1.19	$ 0.86	$ (1.85)	$ 3.91	$ 2.32