Derivatives and Other Financial Instruments - Schedule of Quantitative Information for Level 3 Derivative Contracts (Detail) - Level 3 [Member] - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative Liabilities, Fair value	$ 1,080	$ 838
Energy Contracts [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative Liabilities, Fair value	1,080
Financial Contracts [Member] | Energy Contracts [Member] | Interrelationship Of Forward Energy 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	14
Financial Contracts [Member] | Energy 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	1
Power Contract [Member] | Energy Contracts [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	248
Power Contract [Member] | Energy Contracts [Member] | MWh of Energy Needed to Produce Forecasted Mt of Aluminum at Rate of 17 Million MWh Per Year [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative Liabilities, Fair value	797
Power Contract [Member] | Energy Contracts [Member] | MWh of Energy Needed to Produce Forecasted Mt of Aluminum at Rate of 2 Million Megawatt Hours Per Year [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative Liabilities, Fair value	2
Power Contract [Member] | Energy Contracts [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	$ 18