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Acquisitions and Divestitures (Details) $ in Millions				3 Months Ended		12 Months Ended
Acquisitions and Divestitures (Details) $ in Millions	Mar. 01, 2020 USD ($) employee	Feb. 01, 2020 USD ($) employee	Dec. 01, 2019 USD ($) employee	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)	Jan. 31, 2020 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Restricted cash				$ 52		$ 55
Engineered Products and Forgings \| Small manufacturing facility in the United Kingdom
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Disposal group, consideration							$ 12
Asset impairment charges				12
Engineered Products and Forgings \| Forgings Business
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Disposal group, consideration			$ 64
Asset impairment charges				6
Gain (loss) on sale of business			$ (46)
Disposal group, including discontinued operation, revenue					$ 32
Number of employees \| employee			540
Restricted cash				53		53
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				6		$ 53
Gain (loss) on sale of business				(59)
Disposal group, including discontinued operation, revenue				11	40
Number of employees \| employee		525
Global Rolled Products \| Hard alloy extrusions plant in South Korea
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Disposal group, consideration	$ 62
Gain (loss) on sale of business				27
Disposal group, including discontinued operation, revenue				$ 8	$ 13
Number of employees \| employee	158