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Derivatives and Other Financial Instruments - Additional Information (Detail) t in Thousands | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | ---
Jun. 30, 2016 USD ($) Derivative | Jun. 30, 2015 USD ($) | Jun. 30, 2016 USD ($) MWh Derivative Agency t | Mar. 31, 2016 | Jun. 30, 2015 USD ($) | Dec. 31, 2015 USD ($) MWh t
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 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 | 10 |  | 10 |  |  |
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 |  |  | $ 8,000,000
Fair value of derivatives recorded as liabilities | 11,000,000 |  | 11,000,000 |  |  | $ 58,000,000
Net gain (loss) of derivative instruments | 1,000,000 | $ 15,000,000 | 3,000,000 |  | $ 41,000,000 |
Derivatives Not Designated as Hedging Instruments [Member] | Other Expenses (Income), Net [Member] |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Net gain (loss) of derivative instruments | (6,000,000) | (5,000,000) | (9,000,000) |  | (2,000,000) |
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) | (253,000,000) | 616,000,000 | (368,000,000) |  | 518,000,000 |
Realized gain (loss) on derivatives | $ 1,000,000 | (12,000,000) | $ 5,000,000 |  | (24,000,000) |
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 | 8 |  | 8 |  |  |
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 | 9 |  | 9 |  |  |
Embedded Aluminum Derivative [Member] | Derivatives Designated as Hedging Instruments [Member] |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Aluminum forecast sales | t |  |  | 3,261 |  |  | 3,307
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 gain (loss) expected to be recognized into earnings over the next 12 months |  |  | $ 25,000,000 |  |  |
Derivative instruments ineffectiveness | $ 0 | 0 | 0 |  | 0 |
Embedded Aluminum Derivative [Member] | Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedging [Member] | Other Expenses (Income), Net [Member] |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Other expenses (income), net | 0 | 1,000,000 | 1,000,000 |  | 1,000,000 |
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) | (2,000,000) | $ (8,000,000) |  | (1,000,000) |
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 | 3 |  | 3 |  |  |
Energy Contracts [Member] |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Recognized an unrealized gain (loss) | $ 45,000,000 | (17,000,000) | $ 39,000,000 |  | (11,000,000) |
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 | 3 |  | 3 |  |  |
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 | $ 3,000,000 |  | 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 | 5 |  | 5 |  |  |
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 | $ (2,000,000) | $ (3,000,000) | $ (1,000,000) |  | $ (1,000,000) |
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 |  |  |

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