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Derivative Accounting - Schedule of Gross Notional Amounts Outstanding (Details) - Commodity Contracts MWh in Thousands, MMcf in Thousands | 3 Months Ended
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Mar. 31, 2018 MWh MMcf | Mar. 31, 2017 MWh MMcf
Outstanding gross notional amount of derivatives |  |
Power | MWh | 1,277 | 583
Gas | MMcf | 252 | 240

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