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Derivatives and Other Financial Instruments - Schedule of Fair Values of Level 3 Derivative Instruments Recorded as Assets and Liabilities (Detail) - USD ($) $ in Millions | Sep. 30, 2015 | Dec. 31, 2014
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Derivative Instruments, Gain (Loss) [Line Items] |  |
Fair value asset derivatives | $ 875 | $ 268
Fair value liability derivatives | 260 | 394
Derivatives Designated as Hedging Instruments [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Fair value asset derivatives | 788 | 99
Fair value liability derivatives | 228 | 376
Derivatives Designated as Hedging Instruments [Member] | Prepaid Expenses and Other Current Assets [Member] | Embedded Aluminum Derivative [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Fair value asset derivatives | 70 | 24
Derivatives Designated as Hedging Instruments [Member] | Other Noncurrent Assets [Member] | Embedded Aluminum Derivative [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Fair value asset derivatives | 718 | 73
Derivatives Designated as Hedging Instruments [Member] | Other Noncurrent Assets [Member] | Energy Contracts [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Fair value asset derivatives |  | 2
Derivatives Designated as Hedging Instruments [Member] | Other Current Liabilities [Member] | Embedded Aluminum Derivative [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Fair value liability derivatives | 11 | 24
Derivatives Designated as Hedging Instruments [Member] | Other Noncurrent Liabilities and Deferred Credits [Member] | Embedded Aluminum Derivative [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Fair value liability derivatives | 213 | 352
Derivatives Designated as Hedging Instruments [Member] | Other Noncurrent Liabilities and Deferred Credits [Member] | Energy Contracts [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Fair value liability derivatives | 4 |
Derivatives Not Designated as Hedging Instruments [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Fair value asset derivatives | 87 | 169
Fair value liability derivatives | 32 | 18
Derivatives Not Designated as Hedging Instruments [Member] | Prepaid Expenses and Other Current Assets [Member] | Embedded Aluminum Derivative [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Fair value asset derivatives | 87 | 98
Derivatives Not Designated as Hedging Instruments [Member] | Other Noncurrent Assets [Member] | Embedded Aluminum Derivative [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Fair value asset derivatives |  | 71
Derivatives Not Designated as Hedging Instruments [Member] | Other Current Liabilities [Member] | Embedded Credit Derivative [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Fair value liability derivatives | 4 | 2
Derivatives Not Designated as Hedging Instruments [Member] | Other Noncurrent Liabilities and Deferred Credits [Member] | Embedded Credit Derivative [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Fair value liability derivatives | $ 28 | $ 16

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