Significant Accounting Policies - Additional Information (Detail) $ / shares in Units, shares in Thousands, ¥ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2018 USD ($) $ / shares	Jun. 30, 2018 USD ($) $ / shares	Mar. 31, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($) $ / shares	Sep. 30, 2017 USD ($) $ / shares	Jun. 30, 2017 USD ($) $ / shares	Mar. 31, 2017 USD ($) $ / shares	Dec. 31, 2016 USD ($) $ / shares	Sep. 30, 2018 USD ($) $ / shares shares	Sep. 30, 2017 USD ($) $ / shares	Sep. 30, 2016 USD ($) $ / shares	Sep. 30, 2018 CNY (¥)	Nov. 30, 2017 USD ($)	Sep. 30, 2017 CNY (¥)	Oct. 01, 2015 USD ($)
Significant Accounting Policies [Line Items]
Percentage of inventories utilizing LIFO cost flow assumption					7.00%					7.00%				7.00%
Retained earnings	$ 1,417,000,000				$ 1,707,000,000				$ 1,417,000,000	$ 1,707,000,000
Cost of sales									2,461,000,000	2,054,000,000	$ 1,836,000,000
(Provision) benefit for income taxes									(193,000,000)	(33,000,000)	(33,000,000)
Net income (loss) attributable to Cabot Corporation	$ 94,000,000	$ 88,000,000	$ (173,000,000)	$ (122,000,000)	$ 72,000,000	$ 47,000,000	$ 74,000,000	$ 55,000,000	$ (113,000,000)	$ 248,000,000	$ 147,000,000
Earnings (loss) per common share - basic: | $ / shares	$ 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: | $ / shares	$ 1.51	$ 1.40	$ (2.80)	$ (1.98)	$ 1.13	$ 0.73	$ 1.19	$ 0.86	$ (1.85)	$ 3.91	$ 2.32
Inventories	$ 511,000,000				$ 433,000,000				$ 511,000,000	$ 433,000,000
Deferred income taxes	134,000,000				237,000,000				134,000,000	237,000,000
Equity affiliate investments	52,000,000				56,000,000				52,000,000	56,000,000
Dividends declared									9,000,000	11,000,000	$ 9,000,000
Goodwill	93,000,000				154,000,000				93,000,000	154,000,000
Capitalized interest									2,000,000	1,000,000	1,000,000
Inventory, assets held for rent	5,000,000				5,000,000				5,000,000	5,000,000
Asset retirement obligation reserve	28,000,000				26,000,000				28,000,000	26,000,000
Charges on sale of notes receivables									3,000,000	2,000,000	1,000,000
Changes in allowance									$ 0	0
Chance of utilizing the associated benefit for valuation allowances, maximum									50.00%
China [Member]
Significant Accounting Policies [Line Items]
Accounts and notes receivable, net	$ 5,000,000				11,000,000				$ 5,000,000	11,000,000		¥ 32		¥ 73
Other Nonoperating Income (Expense) [Member]
Significant Accounting Policies [Line Items]
Foreign currency transaction gains (losses)									(4,000,000)	(4,000,000)	(7,000,000)
Accounts Payable and Accrued Liabilities [Member]
Significant Accounting Policies [Line Items]
Non-cash capital expenditures									$ 29,000,000	7,000,000
Minimum [Member] | Buildings [Member]
Significant Accounting Policies [Line Items]
Useful life of property, plant and equipment									20 years
Minimum [Member] | Machinery and Equipment [Member]
Significant Accounting Policies [Line Items]
Useful life of property, plant and equipment									10 years
Minimum [Member] | Other Fixed Assets [Member]
Significant Accounting Policies [Line Items]
Useful life of property, plant and equipment									3 years
Minimum [Member] | Cabot [Member]
Significant Accounting Policies [Line Items]
Equity method investment, ownership percentage	20.00%								20.00%			20.00%
Maximum [Member] | Buildings [Member]
Significant Accounting Policies [Line Items]
Useful life of property, plant and equipment									25 years
Maximum [Member] | Machinery and Equipment [Member]
Significant Accounting Policies [Line Items]
Useful life of property, plant and equipment									25 years
Maximum [Member] | Other Fixed Assets [Member]
Significant Accounting Policies [Line Items]
Useful life of property, plant and equipment									25 years
Maximum [Member] | Cabot [Member]
Significant Accounting Policies [Line Items]
Equity method investment, ownership percentage	50.00%								50.00%			50.00%
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					1,707,000,000					1,707,000,000	1,561,000,000				$ 1,497,000,000
Cost of sales										2,054,000,000	1,836,000,000
(Provision) benefit for income taxes										(33,000,000)	(33,000,000)
Net income (loss) attributable to Cabot Corporation										$ 248,000,000	$ 147,000,000
Earnings (loss) per common share - basic: | $ / shares										$ 3.94	$ 2.34
Earnings (loss) per common share - diluted: | $ / shares										$ 3.91	$ 2.32
Inventories					433,000,000					$ 433,000,000	$ 368,000,000
Deferred income taxes					237,000,000					237,000,000	207,000,000
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					24,000,000					24,000,000	17,000,000				19,000,000
Cost of sales										(11,000,000)	3,000,000
(Provision) benefit for income taxes										(4,000,000)	1,000,000
Net income (loss) attributable to Cabot Corporation										$ 7,000,000	$ (2,000,000)
Earnings (loss) per common share - basic: | $ / shares										$ 0.11	$ (0.04)
Earnings (loss) per common share - diluted: | $ / shares										$ 0.11	$ (0.04)
Inventories					37,000,000					$ 37,000,000	$ 26,000,000
Deferred income taxes					(13,000,000)					(13,000,000)	(9,000,000)
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					1,683,000,000					1,683,000,000	1,544,000,000				$ 1,478,000,000
Cost of sales										2,065,000,000	1,833,000,000
(Provision) benefit for income taxes										(29,000,000)	(34,000,000)
Net income (loss) attributable to Cabot Corporation										$ 241,000,000	$ 149,000,000
Earnings (loss) per common share - basic: | $ / shares										$ 3.83	$ 2.38
Earnings (loss) per common share - diluted: | $ / shares										$ 3.80	$ 2.36
Inventories					396,000,000					$ 396,000,000	$ 342,000,000
Deferred income taxes					$ 250,000,000					$ 250,000,000	$ 216,000,000
No Change in Method of Accounting for Inventories [Member] | Effect of Change [Member]
Significant Accounting Policies [Line Items]
Retained earnings	$ (35,000,000)								$ (35,000,000)
Cost of sales									15,000,000
(Provision) benefit for income taxes									4,000,000
Net income (loss) attributable to Cabot Corporation									$ 11,000,000
Earnings (loss) per common share - basic: | $ / shares									$ (0.19)
Earnings (loss) per common share - diluted: | $ / shares									$ (0.19)
Inventories	(52,000,000)								$ (52,000,000)
Deferred income taxes	$ 17,000,000								$ 17,000,000
Tech Blend and NSCC Carbon (Jiangsu) Co. Ltd. [Member]
Significant Accounting Policies [Line Items]
Business acquisition, number of shares acquired | shares									8,755,329
Tech Blend [Member]
Significant Accounting Policies [Line Items]
Intangible assets													$ 29,000,000
Goodwill													33,000,000
Tech Blend [Member] | Specialty Compounds [Member]
Significant Accounting Policies [Line Items]
Goodwill													$ 33,000,000