Derivatives and Other Financial Instruments - Schedule of Quantitative Information for Level 3 Derivative Contracts (Detail) - Level 3 [Member] - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative Assets, Fair value	$ 9	$ 74
Derivative Liabilities, Fair value	206	$ 615
Energy Contracts [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative Assets, Fair value	2
Derivative Liabilities, Fair value	199
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
Power Related Derivative [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	147
Power Related Derivative [Member] | Energy Contracts [Member] | MWh of Energy Needed to Produce Forecasted Mt of Aluminum at Rate of 11 Million MWh Per Year [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative Liabilities, Fair value	27
Power Related Derivative [Member] | Energy Contracts [Member] | MWh Hours of Energy Needed to Produce the Forecasted Mt of Aluminum at Rate of 2 Million Mwh Hours Per Year [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative Liabilities, Fair value	1
Power Related Derivative [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	$ 24