Significant Accounting Policies - Schedule of Change Retrospectively to Prior Periods Consolidated Statements of Operations (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Significant Accounting Policies [Line Items]
Cost of sales	$ 542	$ 452
Income (loss) from continuing operations before income taxes and equity in earnings of affiliated companies	92	75
(Provision) benefit for income taxes	(205)	(18)
Net income (loss)	(112)	59
Net income (loss) attributable to Cabot Corporation	$ (122)	$ 55
Earnings per common share:
Basic	$ (1.98)	$ 0.87
Diluted	$ (1.98)	$ 0.86
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		$ 452
Income (loss) from continuing operations before income taxes and equity in earnings of affiliated companies		75
(Provision) benefit for income taxes		(18)
Net income (loss)		59
Net income (loss) attributable to Cabot Corporation		$ 55
Earnings per common share:
Basic		$ 0.87
Diluted		$ 0.86
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		$ 454
Income (loss) from continuing operations before income taxes and equity in earnings of affiliated companies		73
(Provision) benefit for income taxes		(17)
Net income (loss)		58
Net income (loss) attributable to Cabot Corporation		$ 54
Earnings per common share:
Basic		$ 0.85
Diluted		$ 0.85
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		$ (2)
Income (loss) from continuing operations before income taxes and equity in earnings of affiliated companies		2
(Provision) benefit for income taxes		(1)
Net income (loss)		1
Net income (loss) attributable to Cabot Corporation		$ 1
Earnings per common share:
Basic		$ 0.02
Diluted		$ 0.01