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, 2025 USD ($) MW-M MWh $ / lb $ / MW
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative Assets, Fair value	$ 40,000,000
Derivative Liabilities, Fair value	1,097,000,000
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	$ 31,000,000
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	73.11
LME forward price	$ 2,598
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	38.35
LME forward price	$ 2,605
Financial Contracts [Member] | Interrelationship of Forward Energy Price and the Contract Price [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative Assets, Fair value	$ 9,000,000	[1]
Financial Contracts [Member] | Interrelationship of Forward Energy Price and the Contract Price [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	6.86	[1]
Financial Contracts [Member] | Interrelationship of Forward Energy Price and the Contract Price [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	40.98	[1]
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	$ 2,598
Midwest aluminum premium | $ / lb	0.7
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	$ 2,600
Midwest aluminum premium | $ / lb	0.7
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	$ 128,000,000
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	$ 2,598
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	2,655
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	$ 969,000,000
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	$ 2,598
Midwest aluminum premium | $ / lb	0.7
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	0.7
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	0.7
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	1.21%
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	5.81%
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	7.02%
Power Contract [Member] | Interrelationship of Forward Energy Price and the Contract Price [Member] | Minimum [Member] | 2025 [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative forward energy volume | MW-M	31	[1]
Power Contract [Member] | Interrelationship of Forward Energy Price and the Contract Price [Member] | Maximum [Member] | 2028 [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative forward energy volume | MW-M	84	[1]

[1] Increases in the power market or in renewable energy production would result in a higher fair value measurement of the derivative asset or a lower fair value measurement of the derivative liability.