Acquisitions and Divestitures - 2018 Divestitures (Details) $ in Millions				3 Months Ended			12 Months Ended
	Dec. 31, 2018 USD ($) employee	Oct. 31, 2018 USD ($)	Apr. 02, 2018 USD ($) employee	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Restructuring and other charges							$ 620	$ 9	$ 165
Gain (loss) on disposition of business							20	109	(57)
Latin America Extrusions (LAE)
Business Acquisition [Line Items]
Balance sheet adjustment	$ 2				$ 2			2
Number of employees | employee			612
Consideration			$ 2
Texarkana, Texas Rolling Mill And Cast House
Business Acquisition [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 disposition of business		$ 154		$ 20
Contingent consideration	5			$ 25	5		25	5
Deferred gain on sale of property	$ 95				95			95
Eger
Business Acquisition [Line Items]
Number of employees | employee	180
Consideration	$ 2				2			2
Gain (loss) on disposition of business					$ (43)		(43)	(43)
Disposed of by Sale | Texarkana, Texas Rolling Mill And Cast House
Business Acquisition [Line Items]
Proceeds from divestiture of businesses		63
Consideration		302
Additional contingent consideration		$ 50
Divested Businesses | Disposed of by Sale | Latin America Extrusions (LAE)
Business Acquisition [Line Items]
Restructuring and other charges						$ 41
Transportation and Construction Solutions | Latin America Extrusions (LAE)
Business Acquisition [Line Items]
Sales								25	$ 115
Engineered Products and Forgings | Eger
Business Acquisition [Line Items]
Sales							$ 32	38
Ta Chen International, Inc. | Texarkana, Texas Rolling Mill And Cast House
Business Acquisition [Line Items]
Period of services performed, processing of metal		6 months
Accounting Standards Update 2016-02 | Property, Plant and Equipment
Business Acquisition [Line Items]
Cumulative Effect on Retained Earnings, Net of Tax								(24)
Accounting Standards Update 2016-02 | Other Noncurrent Liabilities and Deferred Credits
Business Acquisition [Line Items]
Cumulative Effect on Retained Earnings, Net of Tax								119
Accounting Standards Update 2016-02 | Other Noncurrent Assets]
Business Acquisition [Line Items]
Cumulative Effect on Retained Earnings, Net of Tax								$ (22)