Arconic Inc Separation Transaction and Discontinued Operations - Additional Information (Details)				3 Months Ended	6 Months Ended	12 Months Ended
	Mar. 25, 2020 USD ($)	Feb. 01, 2020 USD ($)	Oct. 31, 2018 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Apr. 01, 2020	Feb. 07, 2020 USD ($)	Jan. 01, 2019 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Distribution rate									0.25
Debt issuance costs (C and R)						$ 61,000,000	$ 0	$ 0
Gain (loss) on disposition of business, before tax						(8,000,000)	(63,000,000)	(43,000,000)
Asset impairment charges						5,000,000	442,000,000	9,000,000
Accounting Standards Update 2016-02
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cumulative effect of new accounting											$ 73,000,000
Aluminum Rolling Mill
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Consideration		$ 50,000,000
Gain (loss) on disposition of business, before tax		$ (59,000,000)
Asset impairment charges				$ 6,000,000	$ 53,000,000	6,000,000
Hard Alloy Extrusions Plant In South Korea
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Consideration				62,000,000		62,000,000
Texarkana, Texas Rolling Mill And Cast House
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) on disposition of business, before tax							$ 20,000,000	154,000,000
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
Contingent consideration								$ 5,000,000
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]
Consideration			$ 302,000,000
Proceeds from the sales of businesses			63,000,000
Additional contingent consideration			$ 50,000,000
Lien Notes
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Debt issuance costs (C and R)				$ 45,000,000		$ 45,000,000
Senior Notes | Second-Lien Notes Due 2028
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Aggregate principal amount										$ 600,000,000
Convertible notes, interest rate percentage										6.125%
Line of Credit | Senior Secured First Lien Term B Loan Facility Due 2027 | Secured Debt
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Aggregate principal amount	$ 600,000,000
Debt instrument maturity date	7 years