Derivatives and Other Financial Instruments - Schedule of Quantitative Information for Level 3 Derivative Contracts (Detail) - Energy Contracts [Member] - Level 3 [Member] MWh in Millions	9 Months Ended
	Sep. 30, 2025 USD ($) MWh MW-M $ / MW $ / lb
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative Assets, Fair value	$ 17,000,000
Derivative Liabilities, Fair value	1,165,000,000
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	$ 3,000,000
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	36.6
LME forward price	$ 2,683
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	50.49
LME forward price	$ 2,691
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	$ 14,000,000	[1]
Financial Contracts [Member] | Interrelationship of Forward Energy Price and the Contract Price [Member] | 2025 [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative forward energy price | $ / MW	8.63	[1]
Financial Contracts [Member] | Interrelationship of Forward Energy Price and the Contract Price [Member] | 2028 [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative forward energy price | $ / MW	42.67	[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	$ 2,683
Midwest aluminum premium | $ / lb	0.78
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	$ 2,681
Midwest aluminum premium | $ / lb	0.78
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	$ 123,000,000
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	$ 2,683
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	2,756
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	$ 1,042,000,000
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	$ 2,683
Midwest aluminum premium | $ / lb	0.78
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	$ 2,829
Midwest aluminum premium | $ / lb	0.78
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	$ 3,019
Midwest aluminum premium | $ / lb	0.78
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	0.24%
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	5.61%
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	5.85%
Power Contract [Member] | Interrelationship of Forward Energy Price and the Contract Price [Member] | 2025 [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative forward energy volume | MW-M	74	[1]
Power Contract [Member] | Interrelationship of Forward Energy Price and the Contract Price [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.