Significant Accounting Policies - Schedule of Change Retrospectively to Prior Periods Consolidated Statements of Operations (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Significant Accounting Policies [Line Items]
Cost of sales	$ 654	$ 544	$ 1,824	$ 1,505
Income (loss) from continuing operations before income taxes and equity in earnings of affiliated companies	95	68	16	221
(Provision) benefit for income taxes	4	(16)	(194)	(33)
Net income (loss)	99	55	(176)	194
Net income (loss) attributable to Cabot Corporation	$ 88	$ 47	$ (207)	$ 176
Earnings per common share:
Basic	$ 1.41	$ 0.73	$ (3.36)	$ 2.79
Diluted	$ 1.40	$ 0.73	$ (3.36)	$ 2.78
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		$ 544		$ 1,505
Income (loss) from continuing operations before income taxes and equity in earnings of affiliated companies		68		221
(Provision) benefit for income taxes		(16)		(33)
Net income (loss)		55		194
Net income (loss) attributable to Cabot Corporation		$ 47		$ 176
Earnings per common share:
Basic		$ 0.73		$ 2.79
Diluted		$ 0.73		$ 2.78
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		$ 546		$ 1,509
Income (loss) from continuing operations before income taxes and equity in earnings of affiliated companies		66		217
(Provision) benefit for income taxes		(16)		(32)
Net income (loss)		53		191
Net income (loss) attributable to Cabot Corporation		$ 45		$ 173
Earnings per common share:
Basic		$ 0.71		$ 2.75
Diluted		$ 0.71		$ 2.74
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)		$ (4)
Income (loss) from continuing operations before income taxes and equity in earnings of affiliated companies		2		4
(Provision) benefit for income taxes				(1)
Net income (loss)		2		3
Net income (loss) attributable to Cabot Corporation		$ 2		$ 3
Earnings per common share:
Basic		$ 0.02		$ 0.04
Diluted		$ 0.02		$ 0.04