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Fair Value (Quantitative Information) (Details) $ in Thousands | 9 Months Ended |
--- | --- | ---
Sep. 30, 2018 USD ($) $ / MWH MMBTU MWh $ / MmBtu | Dec. 31, 2017 USD ($)
Power Exchange Agreements [Member] | Surrogate Facility Pricing [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Transaction/Delivery Volumes | MWh | 292,145 |
Average operation and maintenance charges | $ / MWH | 45.61 |
Minimum [Member] | Power Exchange Agreements [Member] | Surrogate Facility Pricing [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value Assumptions Operation And Maintenance Charges | $ / MWH | 40.05 |
Maximum [Member] | Power Exchange Agreements [Member] | Surrogate Facility Pricing [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value Assumptions Operation And Maintenance Charges | $ / MWH | 52.59 |
Fair Value, Measurements, Recurring [Member] | Power Exchange Agreements [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Derivative Liability | $ | $ (5,909) | $ (13,245)
Fair Value, Measurements, Recurring [Member] | Natural Gas Exchange Agreements [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Derivative Liability | $ | $ (3,004) | $ (3,164)
Sales [Member] | Minimum [Member] | Natural Gas Exchange Agreements [Member] | Internally Derived Weighted Average Cost Of Gas [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Derivative, Forward Price | $ / MmBtu | 1.46 |
Transaction/Delivery Volumes | MMBTU | 60,000 |
Sales [Member] | Maximum [Member] | Natural Gas Exchange Agreements [Member] | Internally Derived Weighted Average Cost Of Gas [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Derivative, Forward Price | $ / MmBtu | 3.21 |
Transaction/Delivery Volumes | MMBTU | 310,000 |
Purchase [Member] | Minimum [Member] | Natural Gas Exchange Agreements [Member] | Internally Derived Weighted Average Cost Of Gas [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Derivative, Forward Price | $ / MmBtu | 1.40 |
Transaction/Delivery Volumes | MMBTU | 125,000 |
Purchase [Member] | Maximum [Member] | Natural Gas Exchange Agreements [Member] | Internally Derived Weighted Average Cost Of Gas [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Derivative, Forward Price | $ / MmBtu | 1.83 |
Transaction/Delivery Volumes | MMBTU | 310,000 |

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