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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, 2016 | Dec. 31, 2015
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Derivative Instruments, Gain (Loss) [Line Items] |  |
Fair value asset derivatives | $ 317 | $ 1,137
Fair value liability derivatives | 296 | 208
Derivatives Designated as Hedging Instruments [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Fair value asset derivatives | 317 | 1,068
Fair value liability derivatives | 272 | 173
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 | 40 | 72
Derivatives Designated as Hedging Instruments [Member] | Prepaid Expenses and Other Current Assets [Member] | Energy Contracts [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Fair value asset derivatives | 7 |
Derivatives Designated as Hedging Instruments [Member] | Other Noncurrent Assets [Member] | Embedded Aluminum Derivative [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Fair value asset derivatives | 270 | 994
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 | 25 | 9
Derivatives Designated as Hedging Instruments [Member] | Other Current Liabilities [Member] | Energy Contracts [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Fair value liability derivatives |  | 4
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 | 247 | 160
Derivatives Not Designated as Hedging Instruments [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Fair value asset derivatives |  | 69
Fair value liability derivatives | 24 | 35
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 |  | 69
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 | 6
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 | $ 20 | $ 29

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