v3.26.1
Derivatives and Other Financial Instruments - Schedule of Quantitative Information for Level 3 Derivative Contracts (Detail) - Energy Contracts [Member] - Level 3 [Member]
MWh in Millions
 6 Months Ended
Jun. 30, 2026
USD ($)
MWh
$ / lb
$ / MW
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]  
Derivative Assets, Fair value $ 16,000,000
Derivative Liabilities, Fair value 1,222,000,000
Financial Contracts [Member] | Interrelationship of Forward Energy Price, LME Forward Price and Consumer Price Index [Member]  
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]  
Derivative Liabilities, Fair value $ 22,000,000
Financial Contracts [Member] | Interrelationship of Forward Energy Price, LME Forward Price and Consumer Price Index [Member] | Minimum [Member] | 2026 [Member]  
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]  
Derivative forward energy price for first and last month | $ / MW 45.27
LME forward price for first and last month $ 3,076
Financial Contracts [Member] | Interrelationship of Forward Energy Price, LME Forward Price and Consumer Price Index [Member] | Maximum [Member] | 2026 [Member]  
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]  
Derivative forward energy price for first and last month | $ / MW 27.57
LME forward price for first and last month $ 3,078
Financial Contracts [Member] | Interrelationship of Forward Energy Price and Contract Price and Estimated MW of Renewable Energy Produced [Member]  
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]  
Derivative Assets, Fair value $ 16,000,000
Financial Contracts [Member] | Interrelationship of Forward Energy Price and Contract Price and Estimated MW of Renewable Energy Produced [Member] | 2026 [Member]  
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]  
Derivative forward energy price for first and last month | $ / MW 22.24
LME forward price for first and last month $ 31
Financial Contracts [Member] | Interrelationship of Forward Energy Price and Contract Price and Estimated MW of Renewable Energy Produced [Member] | 2028 [Member]  
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]  
Derivative forward energy price | $ / MW 42.2
LME forward price for first and last month $ 84
Power Contract [Member] | MWh of Energy Needed to Produce Forecasted Mt of Aluminum at Rate of 2 Million MWh Per Year [Member]  
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]  
Derivative Liabilities, Fair value $ 0
Derivative forward energy volume | MWh 2
Power Contract [Member] | MWh of Energy Needed to Produce Forecasted Mt of Aluminum at Rate of 2 Million MWh Per Year [Member] | 2026 [Member]  
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]  
LME forward price for first and last month $ 3,076
Power Contract [Member] | MWh of Energy Needed to Produce Forecasted Mt of Aluminum at Rate of 2 Million MWh Per Year [Member] | 2027 [Member]  
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]  
LME forward price for first and last month 3,083
Power Contract [Member] | MWh of Energy Needed to Produce Forecasted Mt of Aluminum at Rate of 4 Million MWh Per Year [Member]  
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]  
Derivative Liabilities, Fair value $ 99,000,000
Derivative forward energy volume | MWh 4
Power Contract [Member] | MWh of Energy Needed to Produce Forecasted Mt of Aluminum at Rate of 4 Million MWh Per Year [Member] | Minimum [Member] | 2026 [Member]  
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]  
LME forward price for first and last month $ 3,076
Power Contract [Member] | MWh of Energy Needed to Produce Forecasted Mt of Aluminum at Rate of 4 Million MWh Per Year [Member] | Maximum [Member] | 2026 [Member]  
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]  
LME forward price for first and last month 3,049
Power Contract [Member] | MWh of Energy Needed to Produce Forecasted Mt of Aluminum at Rate of 18 Million MWh Per Year [Member]  
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]  
Derivative Liabilities, Fair value $ 1,101,000,000
Derivative forward energy volume | MWh 18
Power Contract [Member] | MWh of Energy Needed to Produce Forecasted Mt of Aluminum at Rate of 18 Million MWh Per Year [Member] | 2026 [Member]  
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]  
LME forward price for first and last month $ 3,076
Midwest aluminum premium for first and last month | $ / lb 1.1998
Power Contract [Member] | MWh of Energy Needed to Produce Forecasted Mt of Aluminum at Rate of 18 Million MWh Per Year [Member] | 2029 [Member]  
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]  
LME forward price for first and last month $ 3,000
Midwest aluminum premium for first and last month | $ / lb 1.13
Power Contract [Member] | MWh of Energy Needed to Produce Forecasted Mt of Aluminum at Rate of 18 Million MWh Per Year [Member] | 2036 [Member]  
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]  
LME forward price for first and last month $ 3,040
Midwest aluminum premium | $ / lb 1.13
Power Contract [Member] | Estimated Spread Between The Respective 30-Year Debt Yield Of Alcoa Corporation And The Counterparty [Member]  
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]  
Derivative Liabilities, Fair value $ 0
Percentage of debt yield credit spread 0.98%
Power Contract [Member] | Estimated Spread Between The Respective 30-Year Debt Yield Of Alcoa Corporation And The Counterparty [Member] | Counterparty [Member]  
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]  
Percentage of debt yield credit spread 5.64%
Power Contract [Member] | Estimated Spread Between The Respective 30-Year Debt Yield Of Alcoa Corporation And The Counterparty [Member] | Alcoa Corporation [Member]  
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]  
Percentage of debt yield credit spread 6.62%