Contingencies and Commitments (Details) $ in Millions				3 Months Ended
	Jan. 01, 2021	Dec. 23, 2020 defendant	Jun. 06, 2019 plaintiff	Mar. 31, 2021 USD ($) location	Dec. 31, 2020 USD ($)	Jun. 26, 2020 USD ($)
Loss Contingencies [Line Items]
Number of cleanup locations | location				30
Remediation reserve balance				$ 10	$ 10
Remediation reserve balance, classified as a current liability				5	5
Payments related to remediation expenses applied against the reserve				1
Expected period of time for decision to be received	6 months
Guarantees of third party related to project financing				33
Combined fair value of guarantees				6	12
Total amount committed under outstanding surety bonds				46
Surety bonds, amount outstanding				25
United Kingdom Litigation
Loss Contingencies [Line Items]
Number of defendants | defendant		23
Behrens et al v Arconic Inc et al
Loss Contingencies [Line Items]
Number of plaintiffs | plaintiff			247
Lehman Brothers International (Europe) (“LBIE”) Claims
Loss Contingencies [Line Items]
Estimate of claim						$ 64
Bank Loan Obligations
Loss Contingencies [Line Items]
Letters of credit, total amount				113
Other Noncurrent Liabilities and Deferred Credits | Separation Agreement
Loss Contingencies [Line Items]
Guarantees of third party related to project financing				1,356	$ 1,398
Alcoa Corporation Workers Compensation Claims
Loss Contingencies [Line Items]
Letters of credit, total amount outstanding				53
Arconic Corporation Environmental Obligations
Loss Contingencies [Line Items]
Letters of credit, total amount outstanding				23
Arconic Corporation Environmental Obligations | Letters Of Credit, Pending Cancellation
Loss Contingencies [Line Items]
Letters of credit, total amount outstanding				$ 6
Maximum | Recurring Costs of Managing Hazardous Substances and Environmental Programs
Loss Contingencies [Line Items]
Percentage of cost of goods sold (percent)				1.00%