Contingencies and Commitments (Details) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2021 USD ($) location	Dec. 31, 2020 USD ($)	Jun. 26, 2020 USD ($) entity
Loss Contingencies [Line Items]
Number of cleanup locations | location		30
Remediation reserve balance	$ 12	$ 12	$ 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	16	16
Combined fair value of guarantees	6	6	12
Total amount committed under outstanding surety bonds	46	46
Surety bonds, amount outstanding	25	25
Bank Loan Obligations
Loss Contingencies [Line Items]
Letters of credit, total amount	116	116
Other Noncurrent Liabilities and Deferred Credits | Separation Agreement
Loss Contingencies [Line Items]
Guarantees of third party related to project financing	1,406	1,406	$ 1,398
Lehman Brothers International (Europe) (“LBIE”) Claims
Loss Contingencies [Line Items]
Number of entities | entity				2
Estimate of claim				$ 64
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	17	17
Arconic Corporation Environmental Obligations | Letters Of Credit, Pending Cancellation
Loss Contingencies [Line Items]
Letters of credit, total amount outstanding	$ 1	$ 1
Maximum | Recurring Costs of Managing Hazardous Substances and Environmental Programs
Loss Contingencies [Line Items]
Percentage of cost of goods sold (percent)		1.00%