Arconic Inc. Separation Transaction and Discontinued Operations - Narrative (Details)				3 Months Ended			6 Months Ended	9 Months Ended
	Apr. 01, 2020 company	Mar. 25, 2020 USD ($)	Feb. 01, 2020 USD ($)	Sep. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	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
Separation, share distribution ratio	0.25
Debt issuance costs incurred								$ 61,000,000	$ 0
Non-cash asset impairments				$ 0		$ 8,000,000		0	$ 442,000,000
Distributions to Arconic Corp				0				(1,255,000,000)
Accumulated other comprehensive loss
Restructuring Cost and Reserve [Line Items]
Distributions to Arconic Corp				(22,000,000)				1,370,000,000
Additional capital
Restructuring Cost and Reserve [Line Items]
Distributions to Arconic Corp				$ 22,000,000				(2,611,000,000)
Aluminum Rolling Mill
Restructuring Cost and Reserve [Line Items]
Disposal group, consideration			$ 50,000,000
Gain on sale of business			$ 59,000,000
Non-cash asset impairments					$ 6,000,000		$ 53,000,000	6,000,000
Hard Alloy Extrusions Plant In South Korea
Restructuring Cost and Reserve [Line Items]
Disposal group, consideration										$ 62,000,000
Lien Notes
Restructuring Cost and Reserve [Line Items]
Debt issuance costs incurred					$ 45,000,000			$ 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 (percentage)											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