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, 2024 USD ($) MWh $ / lb $ / MW
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative Assets, Fair value	$ 1,000,000
Derivative Liabilities, Fair value	1,268,000,000
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	$ 1,000,000
Financial Contracts [Member] | Interrelationship of Forward Energy Price, LME Forward Price and Consumer Price Index [Member] | Minimum [Member] | 2024 [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative forward energy price | $ / MW	36.15
LME forward price	$ 2,620
Financial Contracts [Member] | Interrelationship of Forward Energy Price, LME Forward Price and Consumer Price Index [Member] | Maximum [Member] | 2024 [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative forward energy price | $ / MW	48.99
LME forward price	$ 2,634
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 Assets, Fair value	$ 2,000,000
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] | 2024 [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
LME forward price	$ 2,620
Midwest aluminum premium | $ / lb	0.193
Power Contract [Member] | MWh of Energy Needed to Produce Forecasted Mt of Aluminum at Rate of 2 Million MWh Per Year [Member] | Maximum [Member] | 2024 [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
LME forward price	$ 2,613
Midwest aluminum premium | $ / lb	0.1975
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	$ 175,000,000
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] | 2024 [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
LME forward price	$ 2,620
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	2,748
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	$ 1,089,000,000
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] | 2024 [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
LME forward price	$ 2,620
Midwest aluminum premium | $ / lb	0.193
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	$ 2,795
Midwest aluminum premium | $ / lb	0.23
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	$ 2,995
Midwest aluminum premium | $ / lb	0.23
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	$ 2,000,000
Percentage of debt yield credit spread	1.61%
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	4.98%
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	6.59%