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Derivative Accounting - Schedule of Gross Notional Amounts Outstanding (Details) - Commodity Contracts MWh in Thousands, MMcf in Thousands | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2017 MWh MMcf | Dec. 31, 2016 MWh MMcf
Outstanding gross notional amount of derivatives |  |
Power | MWh | 736 | 1,314
Gas | MMcf | 205 | 194

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