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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
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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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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-SubTopic 20
-Section 55
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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+ References

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