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Fair Value Measurements - Level 3 Quantitative Information (Details) $ in Thousands | 12 Months Ended
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Dec. 31, 2016 USD ($) $ / MMBTU $ / MWh | Dec. 31, 2015 USD ($) $ / MMBTU $ / MWh
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Assets | $ | $ 11,076 | $ 30,364
Liabilities | $ | 58,482 | 63,343
Electricity forward contracts |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Assets | $ | 10,648 | 24,543
Liabilities | $ | $ 32,042 | $ 54,679
Electricity forward contracts | Minimum | Discounted cash flows |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Electricity forward price (per MWh) | $ / MWh | 16.43 | 15.92
Electricity forward contracts | Maximum | Discounted cash flows |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Electricity forward price (per MWh) | $ / MWh | 41.07 | 40.73
Electricity forward contracts | Weighted Average | Discounted cash flows |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Electricity forward price (per MWh) | $ / MWh | 29.86 | 26.86
Option Contracts |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Liabilities | $ |  | $ 5,628
Option Contracts | Minimum | Option model |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Electricity forward price (per MWh) | $ / MWh |  | 23.87
Implied electricity price volatilities (as a percent) |  | 40.00%
Implied natural gas price volatilities (as a percent) |  | 32.00%
Natural gas forward price (per MMbtu) | $ / MMBTU |  |
Option Contracts | Maximum | Option model |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Electricity forward price (per MWh) | $ / MWh |  | 44.13
Implied electricity price volatilities (as a percent) |  | 59.00%
Implied natural gas price volatilities (as a percent) |  | 40.00%
Natural gas forward price (per MMbtu) | $ / MMBTU |  |
Option Contracts | Weighted Average | Option model |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Electricity forward price (per MWh) | $ / MWh |  | 33.91
Implied electricity price volatilities (as a percent) |  | 52.00%
Implied natural gas price volatilities (as a percent) |  | 35.00%
Natural gas forward contracts |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Assets | $ | $ 428 | $ 5,821
Liabilities | $ | $ 26,440 | $ 3,036
Natural gas forward contracts | Minimum | Discounted cash flows |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Natural gas forward price (per MMbtu) | $ / MMBTU | 2.32 | 2.18
Natural gas forward contracts | Maximum | Discounted cash flows |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Natural gas forward price (per MMbtu) | $ / MMBTU | 3.60 | 3.14
Natural gas forward contracts | Weighted Average | Discounted cash flows |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Natural gas forward price (per MMbtu) | $ / MMBTU |  | 2.61
Natural gas forward contracts | Weighted Average | Option model |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Electricity forward price (per MWh) | $ / MWh | 2.81 |

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- Definition Represents the gas price per Metric Million British Thermal Units, used as an input to measure fair value. + References No definition available. + Details
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