Acquisitions and Divestitures (Details) $ in Millions			1 Months Ended	3 Months Ended				9 Months Ended		12 Months Ended
	Oct. 31, 2018 USD ($)	Apr. 02, 2018 USD ($)	Mar. 31, 2017 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Mar. 31, 2017 USD ($) employee	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Restructuring and other charges				$ (2)		$ 19		$ 20	$ 118
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						$ 30		$ 25	$ 86
Cash received on sale of operations		$ 5
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					$ 41
Texarkana, Texas Rolling Mill And Cast House | Subsequent Event
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
Texarkana, Texas Rolling Mill And Cast House | Subsequent Event | 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 years
Texarkana, Texas Rolling Mill And Cast House | Disposed of by Sale | Subsequent Event
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash received on sale of operations	$ 63
Disposal group, consideration	302
Disposal group, consideration consideration	50
Texarkana, Texas rolling mill | Disposed of by Sale | Subsequent Event
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Contingent consideration on sale of assets	4
Texarkana, Texas rolling mill | Disposed of by Sale | Subsequent Event | Scenario, Forecast | Minimum
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on disposal	150
Deferred gain on disposal	85
Texarkana, Texas rolling mill | Disposed of by Sale | Subsequent Event | Scenario, Forecast | Maximum
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on disposal	165
Deferred gain on disposal	$ 100
Slim Aluminium
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash expense related to sale of rolling mill			$ 10
Third party guarantee with a fair value related to environmental remediation			$ 5
Disposal group, including discontinued operation, revenue							$ 37
Number of employees | employee							312
Slim Aluminium | Restructuring and Other Charges
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Loss on sale of rolling mill							$ 60
Third-Party Sales | Building and Construction Systems
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Disposal group, including discontinued operation, revenue										$ 1,070