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Fair Value Measurements - Significant Unobservable Inputs Used to Value Level 3 Instruments (Details 2) - Forward Price $ in Thousands | Jun. 30, 2019 USD ($) $ / MWh | Dec. 31, 2018 USD ($) $ / MWh
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Significant Unobservable Inputs (Level 3) |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Assets | $ | $ 436 | $ 2
Financial and Nonfinancial Liabilities, Fair Value Disclosure | $ | 13,189 | $ 8,216
Electricity forward contracts | Minimum |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Significant Unobservable Input |  | 17.88
Electricity forward contracts | Maximum |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Significant Unobservable Input |  | 37.03
Electricity forward contracts | Significant Unobservable Inputs (Level 3) |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Assets | $ | 294 | $ 0
Financial and Nonfinancial Liabilities, Fair Value Disclosure | $ | $ 11,675 | $ 2,456
Electricity forward contracts | Significant Unobservable Inputs (Level 3) | Weighted-Average |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Significant Unobservable Input | 26.78 | 26.10
Electricity forward contracts | Significant Unobservable Inputs (Level 3) | Minimum |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Significant Unobservable Input | 18.27 |
Electricity forward contracts | Significant Unobservable Inputs (Level 3) | Maximum |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Significant Unobservable Input | 30.11 |
Natural gas contracts | Minimum |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Significant Unobservable Input |  | 1.79
Natural gas contracts | Maximum |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Significant Unobservable Input |  | 2.92
Natural gas contracts | Significant Unobservable Inputs (Level 3) |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Assets | $ | $ 142 | $ 2
Financial and Nonfinancial Liabilities, Fair Value Disclosure | $ | $ 1,514 | $ 5,760
Natural gas contracts | Significant Unobservable Inputs (Level 3) | Weighted-Average |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Significant Unobservable Input | 2.41 | 2.48
Natural gas contracts | Significant Unobservable Inputs (Level 3) | Minimum |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Significant Unobservable Input | (0.23) |
Natural gas contracts | Significant Unobservable Inputs (Level 3) | Maximum |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Significant Unobservable Input | 2.88 |

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