Derivatives and Other Financial Instruments - Schedule of Quantitative Information for Level 3 Derivative Contracts (Detail) - Energy Contracts [Member] - Level 3 [Member] MWh in Millions	3 Months Ended
	Mar. 31, 2025 USD ($) MWh $ / MW $ / lb
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative Assets, Fair value	$ 30,000,000
Derivative Liabilities, Fair value	1,096,000,000
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 Assets, Fair value	$ 30,000,000
Financial Contracts [Member] | Interrelationship of Forward Energy Price, LME Forward Price and Consumer Price Index [Member] | Minimum [Member] | 2025 [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative forward energy price | $ / MW	31.92
LME forward price	$ 2,516
Financial Contracts [Member] | Interrelationship of Forward Energy Price, LME Forward Price and Consumer Price Index [Member] | Maximum [Member] | 2025 [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative forward energy price | $ / MW	60.48
LME forward price	$ 2,547
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 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] | Minimum [Member] | 2025 [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
LME forward price	$ 2,516
Midwest aluminum premium | $ / lb	0.52
Power Contract [Member] | MWh of Energy Needed to Produce Forecasted Mt of Aluminum at Rate of 2 Million MWh Per Year [Member] | Maximum [Member] | 2025 [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
LME forward price	$ 2,530
Midwest aluminum premium | $ / lb	0.52
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	$ 135,000,000
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] | 2025 [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
LME forward price	$ 2,516
Power Contract [Member] | MWh of Energy Needed to Produce Forecasted Mt of Aluminum at Rate of 4 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	2,624
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	$ 958,000,000
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] | 2025 [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
LME forward price	$ 2,516
Midwest aluminum premium | $ / lb	0.52
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	$ 2,701
Midwest aluminum premium | $ / lb	0.52
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	$ 2,888
Midwest aluminum premium | $ / lb	0.52
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	$ 3,000,000
Percentage of debt yield credit spread	1.35%
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	5.35%
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	6.70%