Derivatives and Other Financial Instruments - Schedule of Quantitative Information for Level 3 Derivative Contracts (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2015 USD ($) Index $ / Metric_Ton $ / MWh $ / Barrels $ / AUD $ / lb	Dec. 31, 2014 USD ($)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative Assets, Fair value | $	$ 875	$ 268
Embedded Aluminum Derivative [Member] | Interrelationship of Future Aluminum Prices, Foreign Currency Exchange Rates, and U.S. Consumer Price Index [Member] | Level 3 [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative Assets, Fair value | $	$ 87
Embedded Aluminum Derivative [Member] | Interrelationship of Future Aluminum Prices, Foreign Currency Exchange Rates, and U.S. Consumer Price Index [Member] | Average Price [Member] | Level 3 [Member] | Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Expected future aluminum prices	1,562
Maturity year of future aluminum price	2015
Expected future aluminum prices	1,562
Maturity year of future aluminum price	2015
Embedded Aluminum Derivative [Member] | Interrelationship of Future Aluminum Prices, Foreign Currency Exchange Rates, and U.S. Consumer Price Index [Member] | Average Price [Member] | Level 3 [Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Expected future aluminum prices	1,615
Maturity year of future aluminum price	2016
Expected future aluminum prices	1,615
Maturity year of future aluminum price	2016
Embedded Aluminum Derivative [Member] | Interrelationship of Future Aluminum Prices, Foreign Currency Exchange Rates, and U.S. Consumer Price Index [Member] | Estimated Foreign Currency Exchange Rate [Member] | Level 3 [Member] | Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Foreign currency exchange rate | $ / AUD	0.70
Foreign currency exchange rate expected year	2015
Embedded Aluminum Derivative [Member] | Interrelationship of Future Aluminum Prices, Foreign Currency Exchange Rates, and U.S. Consumer Price Index [Member] | Estimated Foreign Currency Exchange Rate [Member] | Level 3 [Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Foreign currency exchange rate | $ / AUD	0.70
Foreign currency exchange rate expected year	2016
Embedded Aluminum Derivative [Member] | Interrelationship of Future Aluminum Prices, Foreign Currency Exchange Rates, and U.S. Consumer Price Index [Member] | Consumer Price Index [Member] | Level 3 [Member] | Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Expected consumer price index | Index	235
Expected consumer price index, Year	2015
Embedded Aluminum Derivative [Member] | Interrelationship of Future Aluminum Prices, Foreign Currency Exchange Rates, and U.S. Consumer Price Index [Member] | Consumer Price Index [Member] | Level 3 [Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Consumer price index base year	1982
Consumer price index base | Index	100
Expected consumer price index | Index	239
Expected consumer price index, Year	2016
Embedded Aluminum Derivative [Member] | Interrelationship of Future Aluminum and Oil Prices [Member] | Average Price [Member] | Level 3 [Member] | Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Expected future aluminum prices	1,562
Maturity year of future aluminum price	2015
Expected future oil prices | $ / Barrels	47
Maturity year of future oil price	2015
Expected future aluminum prices	1,562
Maturity year of future aluminum price	2015
Embedded Aluminum Derivative [Member] | Interrelationship of Future Aluminum and Oil Prices [Member] | Average Price [Member] | Level 3 [Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Expected future aluminum prices	1,762
Maturity year of future aluminum price	2018
Expected future oil prices | $ / Barrels	60
Maturity year of future oil price	2018
Expected future aluminum prices	1,762
Maturity year of future aluminum price	2018
Embedded Aluminum Derivative [Member] | Price of Aluminum beyond Forward Curve [Member] | Level 3 [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative Assets, Fair value | $	$ 784
Derivative Liabilities, Fair value | $	$ 223
Embedded Aluminum Derivative [Member] | Price of Aluminum beyond Forward Curve [Member] | Average Price [Member] | Level 3 [Member] | Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Expected future aluminum prices	2,160
Maturity year of future aluminum price	2026
Expected future aluminum prices	2,160
Maturity year of future aluminum price	2026
Embedded Aluminum Derivative [Member] | Price of Aluminum beyond Forward Curve [Member] | Average Price [Member] | Level 3 [Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Expected future aluminum prices	2,239
Maturity year of future aluminum price	2027
Expected future aluminum prices	2,239
Maturity year of future aluminum price	2027
Embedded Aluminum Derivative [Member] | Interrelationship of LME Price to Overall Energy Price [Member] | Level 3 [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative Assets, Fair value | $	$ 4
Embedded Aluminum Derivative [Member] | Interrelationship of LME Price to Overall Energy Price [Member] | Average Price [Member] | Level 3 [Member] | Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Expected future aluminum prices	1,560
Maturity year of future aluminum price	2015
Expected future aluminum prices	1,560
Maturity year of future aluminum price	2015
Embedded Aluminum Derivative [Member] | Interrelationship of LME Price to Overall Energy Price [Member] | Average Price [Member] | Level 3 [Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Expected future aluminum prices	1,817
Maturity year of future aluminum price	2019
Expected future aluminum prices	1,817
Maturity year of future aluminum price	2019
Embedded Aluminum Derivative [Member] | Two Contracts [Member] | Price of Aluminum beyond Forward Curve [Member] | Average Price [Member] | Level 3 [Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Expected future aluminum prices	2,347
Maturity year of future aluminum price	2029
Expected future aluminum prices	2,347
Maturity year of future aluminum price	2029
Embedded Aluminum Derivative [Member] | Two Contracts [Member] | Price of Aluminum beyond Forward Curve [Member] | Midwest Premium | Level 3 [Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Foreign currency exchange rate expected year	2029
Embedded Aluminum Derivative [Member] | One Contract [Member] | Price of Aluminum beyond Forward Curve [Member] | Average Price [Member] | Level 3 [Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Expected future aluminum prices	2,643
Maturity year of future aluminum price	2036
Expected future aluminum prices	2,643
Maturity year of future aluminum price	2036
Embedded Aluminum Derivative [Member] | One Contract [Member] | Price of Aluminum beyond Forward Curve [Member] | Midwest Premium | Level 3 [Member] | Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Midwest Premium | $ / lb	0.0795
Foreign currency exchange rate expected year	2021
Embedded Aluminum Derivative [Member] | One Contract [Member] | Price of Aluminum beyond Forward Curve [Member] | Midwest Premium | Level 3 [Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Midwest Premium | $ / lb	0.0795
Foreign currency exchange rate expected year	2036
Embedded Credit Derivative [Member] | Credit Spread between Alcoa and Counterparty [Member] | Level 3 [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative Liabilities, Fair value | $	$ 32
Embedded Credit Derivative [Member] | Credit Spread between Alcoa and Counterparty [Member] | Credit Spread [Member] | Level 3 [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Percentage of credit spread	3.27%
Embedded Credit Derivative [Member] | Credit Spread between Alcoa and Counterparty [Member] | Credit Spread [Member] | Level 3 [Member] | Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Percentage of credit spread	3.04%
Embedded Credit Derivative [Member] | Credit Spread between Alcoa and Counterparty [Member] | Credit Spread [Member] | Level 3 [Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Percentage of credit spread	3.50%
Energy Contracts [Member] | Price of Electricity beyond Forward Curve [Member] | Level 3 [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative Liabilities, Fair value | $	$ 5
Energy Contracts [Member] | Price of Electricity beyond Forward Curve [Member] | Average Price [Member] | Level 3 [Member] | Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Price of electricity beyond forward curve | $ / MWh	45
Maturity date of electricity beyond forward curve	2018
Energy Contracts [Member] | Price of Electricity beyond Forward Curve [Member] | Average Price [Member] | Level 3 [Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Price of electricity beyond forward curve | $ / MWh	121
Maturity date of electricity beyond forward curve	2036