Contingencies and Commitments (Details) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2021 USD ($)	Jun. 30, 2021 USD ($) location	Dec. 31, 2020 USD ($)	Jun. 26, 2020 USD ($) entity
Loss Contingencies [Line Items]
Number of cleanup locations | location		30
Remediation reserve balance	$ 10	$ 10
Remediation reserve balance, classified as a current liability	6	6	$ 5
Payments related to remediation expenses applied against the reserve	(1)
Guarantees of third party related to project financing	18	18
Combined fair value of guarantees	6	6	12
Total amount committed under outstanding surety bonds	46	46
Surety bonds, amount outstanding	25	25
Lehman Brothers International (Europe) (“LBIE”) Claims
Loss Contingencies [Line Items]
Number of entities | entity				2
Estimate of claim				$ 64
Bank Loan Obligations
Loss Contingencies [Line Items]
Letters of credit, total amount	133	133
Other Noncurrent Liabilities and Deferred Credits | Separation Agreement
Loss Contingencies [Line Items]
Guarantees of third party related to project financing	1,435	1,435	$ 1,398
Alcoa Corporation Workers Compensation Claims
Loss Contingencies [Line Items]
Letters of credit, total amount outstanding	53	53
Arconic Corporation Environmental Obligations
Loss Contingencies [Line Items]
Letters of credit, total amount outstanding	23	23
Arconic Corporation Environmental Obligations | Letters Of Credit, Pending Cancellation
Loss Contingencies [Line Items]
Letters of credit, total amount outstanding	$ 6	$ 6
Maximum | Recurring Costs of Managing Hazardous Substances and Environmental Programs
Loss Contingencies [Line Items]
Percentage of cost of goods sold (percent)		1.00%