| **Fair Value Measurements - Significant Unobservable Inputs Used to Value Level 3 Instruments (Details 2) - Forward Price $ in Thousands** | Mar. 31, 2019 USD ($) $ / MWh | Dec. 31, 2018 USD ($) $ / MWh |
|--------------------------------------------------------------------------------------------------------------------------------------------|-------------------------------|-------------------------------|
| Significant Unobservable Inputs (Level 3)                                                                                                  |                          |                          |
| **Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments**          |                          |                          |
| Assets | $                                                                                                                                 | $ 1,076                       | $ 2                           |
| Financial and Nonfinancial Liabilities, Fair Value Disclosure | $                                                                          | 6,688                         | $ 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 | $                                                                                                                                 | 382                           | $ 0                           |
| Financial and Nonfinancial Liabilities, Fair Value Disclosure | $                                                                          | $ 5,206                       | $ 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                                                                                                             | 50.50                         | 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                                                                                                             | 19.77                         |                          |
| 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                                                                                                             | 67.40                         |                          |
| 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 | $                                                                                                                                 | $ 694                         | $ 2                           |
| Financial and Nonfinancial Liabilities, Fair Value Disclosure | $                                                                          | $ 1,482                       | $ 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                                                                                                             | 2.46                          | 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                                                                                                             | 1.90                          |                          |
| 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                                                                                                             | 2.97                          |                          |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Value of input used to measure net derivative asset (liability).
+ References
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| - Definition
Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.
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| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
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| - Definition
Fair value of financial and nonfinancial obligations.
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