Significant Accounting Policies - Additional Information (Detail) $ / shares in Units, $ in Millions		1 Months Ended	3 Months Ended		9 Months Ended
	Oct. 01, 2017 USD ($)	Nov. 30, 2018 USD ($)	Jun. 30, 2018 USD ($) $ / shares	Jun. 30, 2017 USD ($) $ / shares	Jun. 30, 2018 USD ($) Segment $ / shares	Jun. 30, 2017 USD ($) $ / shares	Nov. 30, 2017 USD ($)	Sep. 30, 2017 USD ($)	Oct. 01, 2016 USD ($)	Nov. 30, 2013 USD ($)
Significant Accounting Policies [Line Items]
Goodwill			$ 92		$ 92			$ 154
Percentage of inventories utilizing LIFO cost flow assumption								7.00%
Retained earnings			1,417		1,417			$ 1,707
Cost of sales			654	$ 544	1,824	$ 1,505
(Provision) benefit for income taxes			4	(16)	(194)	(33)
Net income (loss) attributable to Cabot Corporation			$ 88	$ 47	$ (207)	$ 176
Earnings (loss) per common share - basic: | $ / shares			$ 1.41	$ 0.73	$ (3.36)	$ 2.79
Earnings (loss) per common share - diluted: | $ / shares			$ 1.40	$ 0.73	$ (3.36)	$ 2.78
Inventories			$ 482		$ 482			433
Deferred income taxes			68		$ 68			237
Number of business reportable segments | Segment					4
Accounting Standards Update 201609 [Member] | Reclassification of Excess Tax Benefit from Share-Based Compensation Awards from Cash Flows from Financing to Operating Activities [Member]
Significant Accounting Policies [Line Items]
Reclassification of tax benefit from share-based compensation awards						$ 8
Accounting Standards Update 201609 [Member] | (Provision) Benefit for Income Taxes [Member]
Significant Accounting Policies [Line Items]
Discrete tax benefit					$ 2
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]
Retained earnings								1,707	$ 1,561
Cost of sales				$ 544		1,505
(Provision) benefit for income taxes				(16)		(33)
Net income (loss) attributable to Cabot Corporation				$ 47		$ 176
Earnings (loss) per common share - basic: | $ / shares				$ 0.73		$ 2.79
Earnings (loss) per common share - diluted: | $ / shares				$ 0.73		$ 2.78
Inventories								433
Deferred income taxes								237
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]
Retained earnings								24	17
Cost of sales				$ (2)		$ (4)
(Provision) benefit for income taxes						(1)
Net income (loss) attributable to Cabot Corporation				$ 2		$ 3
Earnings (loss) per common share - basic: | $ / shares				$ 0.02		$ 0.04
Earnings (loss) per common share - diluted: | $ / shares				$ 0.02		$ 0.04
Inventories								37
Deferred income taxes								(13)
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]
Retained earnings								1,683	$ 1,544
Cost of sales				$ 546		$ 1,509
(Provision) benefit for income taxes				(16)		(32)
Net income (loss) attributable to Cabot Corporation				$ 45		$ 173
Earnings (loss) per common share - basic: | $ / shares				$ 0.71		$ 2.75
Earnings (loss) per common share - diluted: | $ / shares				$ 0.71		$ 2.74
Inventories								396
Deferred income taxes								$ 250
No Change in Method of Accounting for Inventories [Member] | Effect of Change [Member]
Significant Accounting Policies [Line Items]
Retained earnings			(32)		(32)
Cost of sales			8		11
(Provision) benefit for income taxes			2		3
Net income (loss) attributable to Cabot Corporation			$ 6		$ 8
Earnings (loss) per common share - basic: | $ / shares			$ (0.10)		$ (0.10)
Earnings (loss) per common share - diluted: | $ / shares			$ (0.13)		$ (0.13)
Inventories			$ (48)		$ (48)
Deferred income taxes			$ 16		$ 16
Maximum [Member] | Accounting Standards Update 201609 [Member]
Significant Accounting Policies [Line Items]
Cumulative-effect of net charge to retained earnings	$ (1)
Buildings [Member] | Minimum [Member]
Significant Accounting Policies [Line Items]
Useful life of property, plant and equipment					20 years
Buildings [Member] | Maximum [Member]
Significant Accounting Policies [Line Items]
Useful life of property, plant and equipment					25 years
Machinery and Equipment [Member] | Minimum [Member]
Significant Accounting Policies [Line Items]
Useful life of property, plant and equipment					10 years
Machinery and Equipment [Member] | Maximum [Member]
Significant Accounting Policies [Line Items]
Useful life of property, plant and equipment					25 years
Other Fixed Assets [Member] | Minimum [Member]
Significant Accounting Policies [Line Items]
Useful life of property, plant and equipment					3 years
Other Fixed Assets [Member] | Maximum [Member]
Significant Accounting Policies [Line Items]
Useful life of property, plant and equipment					25 years
Tech Blend [Member]
Significant Accounting Policies [Line Items]
Identifiable intangible assets							$ 29
Goodwill							33
Tech Blend [Member] | Speciality Compounds Reporting Unit [Member]
Significant Accounting Policies [Line Items]
Goodwill							$ 33
NHUMO [Member]
Significant Accounting Policies [Line Items]
Preferred stock redemption amount										$ 25
Dividend percentage of preferred stock					6.00%
Preferred stock dividend					$ 1
NHUMO [Member] | Subsequent Event [Member]
Significant Accounting Policies [Line Items]
Preferred stock dividends due		$ 1