Derivatives and Other Financial Instruments - Schedule of Quantitative Information for Level 3 Derivative Contracts (Detail) $ in Millions	3 Months Ended
	Mar. 31, 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 | $	$ 137	$ 153
Derivative Liabilities, Fair value | $	615	$ 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 | $	$ 137
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	73.13
Maturity date of electricity beyond forward curve	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	55.02
Maturity date of electricity beyond forward curve	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 | $	$ 267
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]
Expected future aluminum prices	1,900
Maturity year of future aluminum price	2019
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]
Expected future aluminum prices	2,445
Maturity year of future aluminum price	2027
Embedded Aluminum Derivative [Member] | Energy Contracts [Member] | Price of Aluminum Beyond Forward Curve [Member] | Level 3 [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative Liabilities, Fair value | $	$ 320
Embedded Aluminum Derivative [Member] | Energy Contracts [Member] | Price of Aluminum Beyond Forward Curve [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]
Expected future aluminum prices	2,533
Maturity month and year of future aluminum price	2029-07
Embedded Aluminum Derivative [Member] | Energy Contracts [Member] | Price of Aluminum Beyond Forward Curve [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	2,553
Maturity month and year of future aluminum price	2029-12
Embedded Aluminum Derivative [Member] | Energy Contracts [Member] | Price of Aluminum Beyond Forward Curve [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]
Midwest Premium | $ / lb	0.1900
Midwest Premium expected year	2019
Embedded Aluminum Derivative [Member] | Energy Contracts [Member] | Price of Aluminum Beyond Forward Curve [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]
Midwest Premium | $ / lb	0.1850
Midwest Premium expected year	2029
Embedded Aluminum Derivative [Member] | Energy Contracts [Member] | Price of Aluminum Beyond Forward Curve [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]
Expected future aluminum prices	2,850
Maturity year of future aluminum price	2036
Embedded Aluminum Derivative [Member] | Energy Contracts [Member] | Price of Aluminum Beyond Forward Curve [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]
Midwest Premium expected year	2036
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	1,900
Maturity month and year of future aluminum price	2019-04
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	1,911
Maturity month and year of future aluminum price	2019-06
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]
Midwest Premium | $ / lb	0.1900
Midwest Premium expected year and month	2019-04
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]
Midwest Premium | $ / lb	0.1900
Midwest Premium expected year and month	2019-06
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 | $	$ 7
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	1,857
Maturity month and year of future aluminum price	2019-04
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	1,954
Maturity month and year of future aluminum price	2019-12
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 | $	$ 21
Percentage of credit spread	3.25%
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	3.96%
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	7.21%