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Derivatives and Other Financial Instruments - Additional Information (Detail) t in Thousands | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Aug. 31, 2016 USD ($) | Sep. 30, 2016 USD ($) Derivative | Sep. 30, 2015 USD ($) | Sep. 30, 2016 USD ($) MWh Derivative Agency t | Sep. 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 |  |
Decrease in unrealized gain |  | $ 4,000,000 | $ (16,000,000) | $ (5,000,000) | $ (19,000,000) |
Other Comprehensive Income (Loss) [Member] |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Decrease in unrealized gain | $ 83,000,000 |  |  |  |  |
Other Noncurrent Assets [Member] |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Decreased derivative asset | 83,000,000 |  |  |  |  |
Level 1 [Member] |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Fair value of derivatives recorded as assets |  | 11,000,000 |  | 11,000,000 |  | $ 8,000,000
Fair value of derivatives recorded as liabilities |  | 9,000,000 |  | 9,000,000 |  | $ 58,000,000
Net gain (loss) of derivative instruments |  | 1,000,000 | 7,000,000 | 4,000,000 | 34,000,000 |
Level 1 [Member] | Other Comprehensive Income (Loss) [Member] |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Net gain (loss) of derivative instruments |  | $ 1,000,000 | 0 | $ 1,000,000 | 0 |
Level 3 [Member] |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Number of derivative instruments | Derivative |  | 10 |  | 10 |  |
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 |  | $ 1,000,000 | (17,000,000) | $ (8,000,000) | (19,000,000) |
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) |  | 462,000,000 | 301,000,000 | 830,000,000 | 819,000,000 |
Realized gain (loss) on derivatives |  | $ 1,000,000 | (1,000,000) | $ (4,000,000) | (25,000,000) |
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 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 |  | $ (7,000,000) | (4,000,000) | $ (15,000,000) | (5,000,000) |
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 |  |
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 |  |  |  | 3,521 |  | 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 |  |  |  | $ 23,000,000 |  |
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 | 2,000,000 |
Energy Contracts [Member] |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Decrease in unrealized gain | $ 84,000,000 |  |  |  |  |
Recognized an unrealized gain (loss) |  | $ 49,000,000 | 3,000,000 | $ 88,000,000 | (8,000,000) |
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 |  |  |  | 2,130,122 |  | 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 |  | $ 8,000,000 | $ (13,000,000) | $ 7,000,000 | $ (14,000,000) |
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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