Acquisitions and Divestitures (Details) $ in Millions				3 Months Ended			12 Months Ended
	Jan. 01, 2019 USD ($)	Oct. 31, 2018 USD ($)	Apr. 02, 2018 USD ($) employee	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2018 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Restructuring and other charges (D)				$ 12	$ 7
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
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 (D)						$ 41
Texarkana, Texas Rolling Mill And Cast House
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
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
Gain (loss) on sale of business		$ 154
Contingent consideration				$ 45			5
Deferred gain on sale of property							$ 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]
Net book value of disposal		$ 63
Cash received on sale of operations		302
Disposal group, consideration 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)