Derivatives and Other Financial Instruments - Schedule of Quantitative Information for Level 3 Derivative Contracts (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) MWh $ / MWh $ / Metric_Ton $ / lb	Dec. 31, 2017 USD ($)
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative Assets, Fair value | $	$ 153	$ 197
Derivative Liabilities, Fair value | $	289	$ 1,173
Derivative D11 [Member] | Energy Contracts [Member] | Interrelationship of Forward Energy Price and the Consumer Price Index and Price of Electricity Beyond Forward Curve [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative Assets, Fair value | $	$ 112
Derivative D11 [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	68.60
Maturity date of electricity beyond forward curve	2019
Derivative D11 [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	44.91
Maturity date of electricity beyond forward curve	2021
Derivative D3 Through D5 [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative Assets, Fair value | $	$ 21
Derivative D3 Through D5 [Member] | Energy Contracts [Member] | Interrelationship of Forward Energy Price and the Consumer Price Index and Price of Electricity Beyond Forward Curve [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative Assets, Fair value | $	$ 20
Derivative D3 Through D5 [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] | 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 | $ / Metric_Ton	2,426
Maturity month and year of future aluminum price	2029-04
Derivative D3 Through D5 [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] | 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 | $ / Metric_Ton	2,458
Maturity month and year of future aluminum price	2029-12
Derivative D3 Through D5 [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] | 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
Derivative D3 Through D5 [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] | 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	0.1800
Midwest Premium expected year	2029
Derivative D3 Through D5 [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] | One Contract [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 | $ / Metric_Ton	2,756
Maturity year of future aluminum price	2036
Derivative D3 Through D5 [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] | 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
Derivative D1 [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]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative Liabilities, Fair value | $	$ 234
Derivative D1 [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]
Megawatt hours per year | MWh	4,000,000
Derivative D1 [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 | $ / Metric_Ton	1,823
Maturity year of future aluminum price	2019
Derivative D1 [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 | $ / Metric_Ton	2,350
Maturity year of future aluminum price	2027
Derivative D8 [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]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative Liabilities, Fair value | $	$ 5
Derivative D8 [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]
Megawatt hours per year | MWh	2,000,000
Derivative D8 [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 | $ / Metric_Ton	1,823
Maturity month and year of future aluminum price	2019-01
Derivative D8 [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 | $ / Metric_Ton	1,848
Maturity month and year of future aluminum price	2019-03
Derivative D8 [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] | 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	2019-01
Derivative D8 [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] | 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	2019-03
Derivative D2 [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 | $	$ 9
Derivative D2 [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 | $ / Metric_Ton	1,946
Maturity month and year of future aluminum price	2019-01
Derivative D2 [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 | $ / Metric_Ton	1,907
Maturity month and year of future aluminum price	2019-12
Derivative D9 [Member] | Energy Contracts [Member] | Estimated Difference In Credit Spread Of Each Of Alcoa Corporation And Counterparty, And Negotiated Multiplier [Member] | Level 3 [Member]
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative Liabilities, Fair value | $	$ 20
Percentage of credit spread	3.11%
Derivative D9 [Member] | Energy Contracts [Member] | Estimated Difference In Credit Spread Of Each Of Alcoa Corporation And Counterparty, And Negotiated Multiplier [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	4.36%
Derivative D9 [Member] | Energy Contracts [Member] | Estimated Difference In Credit Spread Of Each Of Alcoa Corporation And Counterparty, And Negotiated Multiplier [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	7.47%