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Derivatives and Other Financial Instruments - Schedule of Quantitative Information for Level 3 Derivative Contracts (Detail) - Level 3 [Member] - USD ($) $ in Millions | Jun. 30, 2021 | Dec. 31, 2020
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Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |
Derivative Assets, Fair value | $ 5 |
Derivative Liabilities, Fair value | 1,415 | $ 838
Energy Contracts [Member] |  |
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |
Derivative Assets, Fair value | 5 |
Derivative Liabilities, Fair value | 1,415 |
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 Assets, Fair value | 2 |
Financial Contracts [Member] | Energy Contracts [Member] | MWh Of Energy Needed To Produces Forecasted Mt Of Aluminum [Member] |  |
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |
Derivative Assets, Fair value | 3 |
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 | 290 |
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 | 1,105 |
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 |

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