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Fair Value Measurements (Schedule of Gains and Losses Included in the Level 3 Fair Value Category) (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Derivative Instruments Gain Loss [Line Items] |  |  |
Total gains (losses) included in earnings | $ (38) | $ (35) | $ (5)
The amount of total gains (losses) for the period included in earnings attributable to the change in unrealized gains (losses) relating to assets/liabilities still held at the reporting date | 2 | (1) | 2
Operating Revenue |  |  |
Derivative Instruments Gain Loss [Line Items] |  |  |
Total gains (losses) included in earnings | 3 | 0 | 6
The amount of total gains (losses) for the period included in earnings attributable to the change in unrealized gains (losses) relating to assets/liabilities still held at the reporting date | 2 | 0 | 1
Electric fuel and other energy-related purchases |  |  |
Derivative Instruments Gain Loss [Line Items] |  |  |
Total gains (losses) included in earnings | (42) | (35) | (11)
The amount of total gains (losses) for the period included in earnings attributable to the change in unrealized gains (losses) relating to assets/liabilities still held at the reporting date | 0 | (1) | 1
Purchased Gas |  |  |
Derivative Instruments Gain Loss [Line Items] |  |  |
Total gains (losses) included in earnings | 1 | 0 | 0
The amount of total gains (losses) for the period included in earnings attributable to the change in unrealized gains (losses) relating to assets/liabilities still held at the reporting date | $ 0 | $ 0 | $ 0

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- Definition This element represents the amount of gains (losses) for the period included in earnings attributable to the change in unrealized gains/losses relating to assets still held at the reporting date for assets and liabilities measured at fair value on a recurring basis with a significant amount of unobservable inputs. + References No definition available. + Details
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- Definition This element represents the net amount of realized and unrealized gains or (losses) included in earnings for assets and liabilities measured at fair value on a recurring basis and included in the Level 3 fair value category. + References No definition available. + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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