Significant Accounting Policies - Schedule of Change Retrospectively to Prior Periods Consolidated Statements of Operations (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
Significant Accounting Policies [Line Items]
Cost of sales									$ 2,461	$ 2,054	$ 1,836
Income (loss) from continuing operations before income taxes and equity in earnings of affiliated companies									117	299	191
(Provision) benefit for income taxes									(193)	(33)	(33)
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 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
Diluted	$ 1.51	$ 1.40	$ (2.80)	$ (1.98)	$ 1.13	$ 0.73	$ 1.19	$ 0.86	$ (1.85)	$ 3.91	$ 2.32
Change in Method of Accounting for Inventories from Last-in, First-out Method to First-in, First-out Method [Member]
Significant Accounting Policies [Line Items]
Cost of sales										$ 2,054	$ 1,836
Income (loss) from continuing operations before income taxes and equity in earnings of affiliated companies										299	191
(Provision) benefit for income taxes										(33)	(33)
Net income (loss)										273	162
Net income (loss) attributable to Cabot Corporation										$ 248	$ 147
Earnings per common share:
Basic										$ 3.94	$ 2.34
Diluted										$ 3.91	$ 2.32
Change in Method of Accounting for Inventories from Last-in, First-out Method to First-in, First-out Method [Member] | As Originally Reported [Member]
Significant Accounting Policies [Line Items]
Cost of sales										$ 2,065	$ 1,833
Income (loss) from continuing operations before income taxes and equity in earnings of affiliated companies										288	194
(Provision) benefit for income taxes										(29)	(34)
Net income (loss)										266	164
Net income (loss) attributable to Cabot Corporation										$ 241	$ 149
Earnings per common share:
Basic										$ 3.83	$ 2.38
Diluted										$ 3.80	$ 2.36
Change in Method of Accounting for Inventories from Last-in, First-out Method to First-in, First-out Method [Member] | Effect of Change [Member]
Significant Accounting Policies [Line Items]
Cost of sales										$ (11)	$ 3
Income (loss) from continuing operations before income taxes and equity in earnings of affiliated companies										11	(3)
(Provision) benefit for income taxes										(4)	1
Net income (loss)										7	(2)
Net income (loss) attributable to Cabot Corporation										$ 7	$ (2)
Earnings per common share:
Basic										$ 0.11	$ (0.04)
Diluted										$ 0.11	$ (0.04)