Derivatives and Other Financial Instruments - Schedule of Quantitative Information for Level 3 Derivative Contracts (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2019 USD ($) MWh $ / MWh $ / Metric_Ton $ / lb	Dec. 31, 2018 USD ($)
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative Assets, Fair value | $	$ 119	$ 153
Derivative Liabilities, Fair value | $	427	$ 289
Financial Contracts [Member] | Energy Contracts [Member] | Interrelationship of Forward Energy Price and the Consumer Price Index and Price of Electricity Beyond Forward Curve [Member] | Level 3 [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative Assets, Fair value | $	$ 119
Financial Contracts [Member] | Energy Contracts [Member] | Interrelationship of Forward Energy Price and the Consumer Price Index and Price of Electricity Beyond Forward Curve [Member] | Level 3 [Member] | Minimum [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Price of electricity beyond forward curve | $ / MWh	70.04
Maturity date of contract	2019
Financial Contracts [Member] | Energy Contracts [Member] | Interrelationship of Forward Energy Price and the Consumer Price Index and Price of Electricity Beyond Forward Curve [Member] | Level 3 [Member] | Maximum [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Price of electricity beyond forward curve | $ / MWh	55.30
Maturity date of contract	2021
Embedded Aluminum Derivative [Member] | Energy Contracts [Member] | Interrelationship of LME Price to Amount of Megawatt Hours of Energy Needed to Produce Forecasted Metric Tons of Aluminum [Member] | Level 3 [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative Liabilities, Fair value | $	$ 201
Megawatt hours per year | MWh	4,000,000
Embedded Aluminum Derivative [Member] | Energy Contracts [Member] | Interrelationship of LME Price to Amount of Megawatt Hours of Energy Needed to Produce Forecasted Metric Tons of Aluminum [Member] | Level 3 [Member] | Minimum [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Maturity date of contract	2019
Expected future aluminum prices	1,708
Embedded Aluminum Derivative [Member] | Energy Contracts [Member] | Interrelationship of LME Price to Amount of Megawatt Hours of Energy Needed to Produce Forecasted Metric Tons of Aluminum [Member] | Level 3 [Member] | Maximum [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Maturity date of contract	2027
Expected future aluminum prices	2,257
Embedded Aluminum Derivative [Member] | Energy Contracts [Member] | Interrelationship of LME and Midwest Premium Price to Amount of Megawatt Hours of Energy Needed to Produce Forecasted Metric Tons of Aluminum [Member] | Level 3 [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative Liabilities, Fair value | $	$ 206
Megawatt hours per year | MWh	11,000,000
Embedded Aluminum Derivative [Member] | Energy Contracts [Member] | Interrelationship of LME and Midwest Premium Price to Amount of Megawatt Hours of Energy Needed to Produce Forecasted Metric Tons of Aluminum [Member] | Level 3 [Member] | Two Contracts [Member] | Midwest Premium [Member] | Minimum [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Maturity date of contract	2019
Midwest Premium | $ / lb	0.1800
Embedded Aluminum Derivative [Member] | Energy Contracts [Member] | Interrelationship of LME and Midwest Premium Price to Amount of Megawatt Hours of Energy Needed to Produce Forecasted Metric Tons of Aluminum [Member] | Level 3 [Member] | Two Contracts [Member] | Midwest Premium [Member] | Maximum [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Maturity date of contract	2029
Midwest Premium | $ / lb	0.1900
Embedded Aluminum Derivative [Member] | Energy Contracts [Member] | Interrelationship of LME and Midwest Premium Price to Amount of Megawatt Hours of Energy Needed to Produce Forecasted Metric Tons of Aluminum [Member] | Level 3 [Member] | Two Contracts [Member] | Average Price [Member] | Minimum [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Maturity date of contract	2019
Expected future aluminum prices	1,708
Embedded Aluminum Derivative [Member] | Energy Contracts [Member] | Interrelationship of LME and Midwest Premium Price to Amount of Megawatt Hours of Energy Needed to Produce Forecasted Metric Tons of Aluminum [Member] | Level 3 [Member] | Two Contracts [Member] | Average Price [Member] | Maximum [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Expected future aluminum prices	2,365
Maturity year of contract	2029-12
Embedded Aluminum Derivative [Member] | Energy Contracts [Member] | Interrelationship of LME and Midwest Premium Price to Amount of Megawatt Hours of Energy Needed to Produce Forecasted Metric Tons of Aluminum [Member] | Level 3 [Member] | One Contract [Member] | Midwest Premium [Member] | Maximum [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Maturity date of contract	2036
Embedded Aluminum Derivative [Member] | Energy Contracts [Member] | Interrelationship of LME and Midwest Premium Price to Amount of Megawatt Hours of Energy Needed to Produce Forecasted Metric Tons of Aluminum [Member] | Level 3 [Member] | One Contract [Member] | Average Price [Member] | Maximum [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Maturity date of contract	2036
Expected future aluminum prices	2,660
Embedded Aluminum Derivative [Member] | Energy Contracts [Member] | Interrelationship of LME Price to Overall Energy Price [Member] | Level 3 [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative Liabilities, Fair value | $	$ 2
Embedded Aluminum Derivative [Member] | Energy Contracts [Member] | Interrelationship of LME Price to Overall Energy Price [Member] | Level 3 [Member] | Average Price [Member] | Minimum [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Expected future aluminum prices	1,847
Maturity year of contract	2019-10
Embedded Aluminum Derivative [Member] | Energy Contracts [Member] | Interrelationship of LME Price to Overall Energy Price [Member] | Level 3 [Member] | Average Price [Member] | Maximum [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Expected future aluminum prices	1,847
Maturity year of contract	2019-12
Embedded Aluminum Derivative [Member] | Energy Contracts [Member] | Interrelationship of LME Price to Amount of Megawatt Hours of Energy Needed to Produce Forecasted Metric Tons of Aluminum One [Member] | Level 3 [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Megawatt hours per year | MWh	2,000,000
Embedded Aluminum Derivative [Member] | Energy Contracts [Member] | Interrelationship of LME Price to Amount of Megawatt Hours of Energy Needed to Produce Forecasted Metric Tons of Aluminum One [Member] | Level 3 [Member] | Minimum [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Expected future aluminum prices	1,708
Maturity year of contract	2019-10
Embedded Aluminum Derivative [Member] | Energy Contracts [Member] | Interrelationship of LME Price to Amount of Megawatt Hours of Energy Needed to Produce Forecasted Metric Tons of Aluminum One [Member] | Level 3 [Member] | Maximum [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Expected future aluminum prices	1,728
Maturity year of contract	2019-12
Embedded Aluminum Derivative [Member] | Energy Contracts [Member] | Interrelationship of LME Price to Amount of Megawatt Hours of Energy Needed to Produce Forecasted Metric Tons of Aluminum One [Member] | Level 3 [Member] | Midwest Premium [Member] | Minimum [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Maturity year of contract	2019-10
Midwest Premium | $ / lb	0.1800
Embedded Aluminum Derivative [Member] | Energy Contracts [Member] | Interrelationship of LME Price to Amount of Megawatt Hours of Energy Needed to Produce Forecasted Metric Tons of Aluminum One [Member] | Level 3 [Member] | Midwest Premium [Member] | Maximum [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Maturity year of contract	2019-12
Midwest Premium | $ / lb	0.1900
Embedded Credit Derivative [Member] | Energy Contracts [Member] | Estimated Spread Between the Respective 30-year Debt Yield of Alcoa Corporation and the Counterparty [Member] | Level 3 [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative Liabilities, Fair value | $	$ 18
Percentage of credit spread	3.08%
Embedded Credit Derivative [Member] | Energy Contracts [Member] | Estimated Spread Between the Respective 30-year Debt Yield of Alcoa Corporation and the Counterparty [Member] | Level 3 [Member] | Counterparty [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Percentage of credit spread	3.36%
Embedded Credit Derivative [Member] | Energy Contracts [Member] | Estimated Spread Between the Respective 30-year Debt Yield of Alcoa Corporation and the Counterparty [Member] | Level 3 [Member] | Alcoa Corporation [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Percentage of credit spread	6.44%