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Fair Value (Quantitative Information) (Details) $ / shares in Units, $ in Thousands | 3 Months Ended |
--- | --- | ---
Mar. 31, 2018 USD ($) $ / MWH MMBTU MWh $ / shares $ / MmBtu | Dec. 31, 2017 USD ($)
Power Exchange Agreements [Member] | Surrogate Facility Pricing [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Average operation and maintenance charges | $ / MWH | 41.95 |
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 | 38.87 |
Transaction/Delivery Volumes | MWh | 42,682 |
Minimum [Member] | Power Option Agreement [Member] | Black Scholes Merton [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Transaction/Delivery Volumes | MWh | 94,221 |
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 | 45.20 |
Transaction/Delivery Volumes | MWh | 396,984 |
Maximum [Member] | Power Option Agreement [Member] | Black Scholes Merton [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Transaction/Delivery Volumes | MWh | 128,491 |
Fair Value, Measurements, Recurring [Member] | Power Exchange Agreements [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Derivative Liability | $ | $ (10,163) | $ (13,245)
Fair Value, Measurements, Recurring [Member] | Power Option Agreement [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Derivative Liability | $ | (5) | (19)
Fair Value, Measurements, Recurring [Member] | Natural Gas Exchange Agreements [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Derivative Liability | $ | $ (2,805) | $ (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 | 1.33 |
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 | 2.85 |
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 | 1.41 |
Transaction/Delivery Volumes | MMBTU | 115,000 |
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 | 2.00 |
Transaction/Delivery Volumes | MMBTU | 310,000 |
2019 | Power Option Agreement [Member] | Black Scholes Merton [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value Assumptions, Exercise Price | $ / shares | $ 40.42 |
2018 to 2019 [Member] | Power Exchange Agreements [Member] | Surrogate Facility Pricing [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Escalation Factor | 5.00% |
2018 | Power Option Agreement [Member] | Black Scholes Merton [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value Assumptions, Exercise Price | $ / shares | $ 34.01 |

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