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Derivatives and Other Financial Instruments - Additional Information (Detail) t in Thousands, BRL in Millions | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Jul. 31, 2014 Plant Contract | Sep. 30, 2015 USD ($) Derivative | Sep. 30, 2014 USD ($) | Sep. 30, 2015 USD ($) MWh Derivative Agency t | Sep. 30, 2014 USD ($) | Dec. 31, 2014 USD ($) MWh t | Sep. 30, 2015 BRL Derivative
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Other derivative contracts estimated term of quoted market prices, in years |  |  |  | 10 years |  |  |
Number of contracts | Contract | 2 |  |  |  |  |  |
Number of smelters | Plant | 2 |  |  |  |  |  |
Number of credit rating agencies | Agency |  |  |  | 3 |  |  |
Level 3 [Member] |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Number of derivative instruments | Derivative |  | 9 |  | 9 |  |  | 9
Level 1 [Member] |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Fair value of derivatives recorded as assets |  | $ 6,000,000 |  | $ 6,000,000 |  | $ 2,000,000 |
Fair value of derivatives recorded as liabilities |  | 54,000,000 |  | 54,000,000 |  | $ 31,000,000 |
Net gain (loss) of derivative instruments |  | (7,000,000) | $ (15,000,000) | 34,000,000 | $ (2,000,000) |  |
Derivatives Not Designated as Hedging Instruments [Member] | Other (income) expenses, net |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Net gain (loss) of derivative instruments |  | (17,000,000) | (21,000,000) | (19,000,000) | (15,000,000) |  |
Foreign Exchange Contracts Two [Member] |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Loss recognized on forward contract |  |  |  | 4,000,000 |  |  |
Foreign Exchange Contracts Two [Member] | Forward Contracts [Member] |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Forward contract to purchase to mitigate the foreign currency risk related to a Canadian/Brazilian-denominated loan due in 2014 |  | 231,000,000 |  | $ 231,000,000 |  |  | BRL 543
Maturity date of forward contract |  |  |  | Mar. 31, 2014 |  |  |
Embedded Aluminum Derivative [Member] | Cash Flow Hedging [Member] |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Recognized an unrealized gain (loss) |  | 301,000,000 | (4,000,000) | $ 819,000,000 | (28,000,000) |  |
Realized gain (loss) on derivatives |  | $ (1,000,000) | (8,000,000) | $ (25,000,000) | (19,000,000) |  |
Embedded Aluminum Derivative [Member] | Level 3 [Member] |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Number of derivative instruments | Derivative |  | 7 |  | 7 |  |  | 7
Embedded Aluminum Derivative [Member] | Level 3 [Member] | Cash Flow Hedging [Member] |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Number of derivative instruments | Derivative |  | 5 |  | 5 |  |  | 5
Embedded Aluminum Derivative [Member] | Derivatives Not Designated as Hedging Instruments [Member] |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Net gain (loss) of derivative instruments |  | $ (4,000,000) | (11,000,000) | $ (5,000,000) | (12,000,000) |  |
Embedded Aluminum Derivative [Member] | Derivatives Not Designated as Hedging Instruments [Member] | Level 3 [Member] |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Number of derivative instruments | Derivative |  | 2 |  | 2 |  |  | 2
Embedded Aluminum Derivative [Member] | Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedging [Member] |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Amount of loss expected to be recognized into earnings over the next 12 months |  |  |  | $ 41,000,000 |  |  |
Derivative instruments ineffectiveness |  | $ 0 |  | 0 | 0 |  |
Embedded Aluminum Derivative [Member] | Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedging [Member] | Other (income) expenses, net |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Other expenses (income), net |  |  |  | $ 2,000,000 | (1,000,000) |  |
Embedded Aluminum Derivative [Member] | Derivatives Designated as Hedging Instruments [Member] | Maximum [Member] | Cash Flow Hedging [Member] |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Derivative instruments ineffectiveness |  |  | 0 |  |  |  |
Embedded Aluminum Derivative [Member] | Derivatives Designated as Hedging Instruments [Member] | Maximum [Member] | Cash Flow Hedging [Member] | Other (income) expenses, net |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Other expenses (income), net |  | 1,000,000 | (1,000,000) |  |  |  |
Embedded Aluminum Derivative [Member] | Derivatives Designated as Hedging Instruments [Member] | Scenario, Forecast [Member] |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Aluminum forecast sales | t |  |  |  | 3,369 |  | 3,610 |
Energy Contracts [Member] |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Recognized an unrealized gain (loss) |  | $ 3,000,000 | (28,000,000) | $ (8,000,000) | (10,000,000) |  |
Energy Contracts [Member] | Level 3 [Member] | Cash Flow Hedging [Member] |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Number of derivative instruments | Derivative |  | 1 |  | 1 |  |  | 1
Energy Contracts [Member] | Derivatives Designated as Hedging Instruments [Member] |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Derivative instruments ineffectiveness |  | $ 0 | 0 | $ 0 | 0 |  |
Forecasted energy purchases in megawatt hours | MWh |  |  |  | 59,409,328 |  | 59,409,328 |
Embedded Credit Derivative [Member] | Level 3 [Member] |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Number of derivative instruments | Derivative |  | 1 |  | 1 |  |  | 1
Embedded Credit Derivative [Member] | Derivatives Not Designated as Hedging Instruments [Member] |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Net gain (loss) of derivative instruments |  | $ (13,000,000) | $ (10,000,000) | $ (14,000,000) | $ (3,000,000) |  |
Embedded Credit Derivative [Member] | Derivatives Not Designated as Hedging Instruments [Member] | Level 3 [Member] |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Number of derivative instruments | Derivative |  | 0 |  | 0 |  |  | 0
Power Contract One [Member] |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Power contracts maturity date | Jun. 30, 2014 |  |  |  |  |  |
Power Contract Two [Member] |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Power contracts maturity date | Jun. 30, 2014 |  |  |  |  |  |

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