Acquisitions and Divestitures (Details) $ in Millions					3 Months Ended							9 Months Ended		12 Months Ended
	Aug. 15, 2019 USD ($)	Jan. 01, 2019 USD ($)	Oct. 31, 2018 USD ($)	Apr. 02, 2018 USD ($) employee	Mar. 31, 2020 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Oct. 30, 2019 USD ($)	Sep. 13, 2019 USD ($)	Aug. 23, 2019 USD ($)	May 31, 2019 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Restructuring and other charges (benefits)						$ 119			$ (2)			$ 630	$ 20
Latin America Extrusions (LAE)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Disposal group, consideration				$ 2
Purchase price allocation, subsequent years, remaining adjustments								$ 2						$ 2
Number of employees | employee				612
Latin America Extrusions (LAE) | Disposed of by Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Disposal group, including discontinued operation, revenue										$ 25
Latin America Extrusions (LAE) | Disposed of by Sale | Divested Businesses
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Restructuring and other charges (benefits)											$ 41
Texarkana, Texas Rolling Mill And Cast House
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) on sale of business								154
Period of operationalizing rolling mill equipment of transaction closing date			36 months
Minimum required terms of leases on cast house building and equipment			18 months
Period of supply with aluminum			24 months
Contingent consideration						45		5				$ 45		5
Deferred gain on sale of property								$ 95						95
Texarkana, Texas Rolling Mill And Cast House | Ta Chen International, Inc.
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Period of services performed, processing of metal			6 months
Texarkana, Texas Rolling Mill And Cast House | Disposed of by Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash received on sale of operations			$ 302
Net book value of disposal			63
Disposal group, consideration			$ 50
Accounting Standards Update 2016-02 | Property, Plant and Equipment
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Effect on retained earnings														(24)
Accounting Standards Update 2016-02 | Other Noncurrent Liabilities and Deferred Credits
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Effect on retained earnings														119
Accounting Standards Update 2016-02 | Other Noncurrent Assets
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Effect on retained earnings		$ (22)												$ (22)
Arconic Engineered Structures | Small Additive Manufacturing Facility
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Disposal group, consideration																		$ 1
Gain (loss) on sale of business							$ (13)
Cash received on sale of operations	$ 13
Asset impairment charges							$ 9
Global Rolled Products | Aluminum Rolling Mill
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Disposal group, consideration																	$ 50
Asset impairment charges						59
Engineered Products And Forgings | Forgings Business
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Disposal group, consideration																$ 62
Asset impairment charges						$ 43
Subsequent Event | Global Rolled Products | Alloy Extrusions
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Disposal group, consideration															$ 61
Scenario, Forecast | Minimum | Global Rolled Products | Alloy Extrusions
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) on sale of business					$ 20
Scenario, Forecast | Maximum | Global Rolled Products | Alloy Extrusions
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) on sale of business					$ 25