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Fair Value Measurements - Significant Unobservable Inputs Used to Value Level 3 Instruments (Details 2) $ in Thousands | Jun. 30, 2018 USD ($) $ / MWh $ / MMBTU | Dec. 31, 2017 USD ($) $ / MWh $ / MMBTU
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Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Liabilities | $ 95,443 | $ 96,422
Significant Unobservable Inputs (Level 3) |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Assets | 2,071 | 1,036
Liabilities | 11,429 | 19,292
Significant Unobservable Inputs (Level 3) | Electricity forward contracts |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Assets | 0 | 21
Liabilities | 1,928 | 15,485
Significant Unobservable Inputs (Level 3) | Option Contracts |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Assets | 1,410 |
Liabilities | 203 |
Significant Unobservable Inputs (Level 3) | Natural gas contracts |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Assets | 661 | 1,015
Liabilities | $ 9,298 | $ 3,807
Discounted cash flows | Forward Price | Electricity forward contracts | Weighted Average |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Significant Unobservable Input | $ / MWh |  | 27.89
Discounted cash flows | Forward Price | Natural gas contracts | Weighted Average |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Significant Unobservable Input | $ / MWh |  | 2.71
Discounted cash flows | Significant Unobservable Inputs (Level 3) | Forward Price | Electricity forward contracts | Minimum |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Significant Unobservable Input | $ / MWh | 20.43 | 18.51
Discounted cash flows | Significant Unobservable Inputs (Level 3) | Forward Price | Electricity forward contracts | Maximum |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Significant Unobservable Input | $ / MWh | 63.14 | 38.75
Discounted cash flows | Significant Unobservable Inputs (Level 3) | Forward Price | Electricity forward contracts | Weighted Average |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Significant Unobservable Input | $ / MWh | 60.68 |
Discounted cash flows | Significant Unobservable Inputs (Level 3) | Forward Price | Natural gas contracts | Minimum |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Significant Unobservable Input | $ / MMBTU | 1.65 | 2.33
Discounted cash flows | Significant Unobservable Inputs (Level 3) | Forward Price | Natural gas contracts | Maximum |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Significant Unobservable Input | $ / MMBTU | 2.82 | 3.11
Discounted cash flows | Significant Unobservable Inputs (Level 3) | Forward Price | Natural gas contracts | Weighted Average |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Significant Unobservable Input | $ / MWh | 2.32 |
Option model | Significant Unobservable Inputs (Level 3) | Forward Price | Option Contracts | Minimum |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Significant Unobservable Input | 0.93 |
Option model | Significant Unobservable Inputs (Level 3) | Forward Price | Option Contracts | Maximum |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Significant Unobservable Input | 1.10 |
Option model | Significant Unobservable Inputs (Level 3) | Electricity price volatilities | Option Contracts | Weighted Average |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Significant Unobservable Input | 1.01 |

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