Acquisitions and Divestitures (Details) $ in Millions		1 Months Ended				3 Months Ended		6 Months Ended	9 Months Ended
	Mar. 01, 2020 USD ($) employee	Jun. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)	May 31, 2020 USD ($)	Feb. 28, 2020 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Feb. 01, 2020 USD ($) employee
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total sales						$ 1,890	$ 1,415		$ 5,366	$ 4,213
Disposed of by sale | Aluminum Rolling Mill in Itapissuma, Brazil
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Consideration received											$ 46
Payments To Settle Working Capital And Other Adjustments Related To Disposal		$ 2	$ 4
Gain (loss) on sale						60
Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down					$ 6			$ 53
Proceeds from Divestiture of Businesses						$ 1
Total sales								$ 143
Number of employees | employee											500
Disposed of by sale | Hard Alloy Extrusions Plant in South Korea
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Consideration received	$ 55
Gain (loss) on sale	31
Proceeds from Divestiture of Businesses				$ 1
Total sales										$ 51
Write-off of goodwill	$ 6
Number of employees | employee	160