Arconic Inc. Separation Transaction and Discontinued Operations - Narrative (Details)				3 Months Ended	6 Months Ended
	Apr. 01, 2020 company	Mar. 25, 2020 USD ($)	Feb. 01, 2020 USD ($)	Mar. 31, 2020 USD ($)	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Mar. 01, 2020 USD ($)	Feb. 07, 2020 USD ($)
Restructuring Cost and Reserve [Line Items]
Number of independent, publicly-traded companies resulting from proposed separation transaction | company	2
Debt issuance costs incurred					$ 1,000,000	$ 61,000,000
Separation Transaction, Distribution Rate	0.25
Aluminum Rolling Mill
Restructuring Cost and Reserve [Line Items]
Consideration			$ 50,000,000
Gain (loss) on sale of business			$ (59,000,000)	$ 6,000,000			$ 53,000,000
Hard Alloy Extrusions Plant In South Korea
Restructuring Cost and Reserve [Line Items]
Consideration								$ 62,000,000
Gain (loss) on sale of business				27,000,000
Lien Notes
Restructuring Cost and Reserve [Line Items]
Debt issuance costs incurred				$ 45,000,000
Senior Notes | Second Lien Notes Due 2028
Restructuring Cost and Reserve [Line Items]
Aggregate principal amount									$ 600,000,000
Long-term debt, interest rate									6.125%
Line of Credit | Senior Secured First Lien Term B Loan Facility Due 2027 | Secured Debt
Restructuring Cost and Reserve [Line Items]
Aggregate principal amount		$ 600,000,000
Credit agreement term		7 years