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Fair Value Measurements - Significant Unobservable Inputs Used to Value Level 3 Instruments (Details 2) $ in Thousands | 3 Months Ended | 12 Months Ended
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Mar. 31, 2018 USD ($) $ / MMBTU $ / MWh | Dec. 31, 2017 USD ($) $ / MMBTU $ / MWh
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Assets | $ 1,548 | $ 1,036
Liabilities | 21,302 | 19,292
Electricity forward contracts |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Assets | 1,028 | 21
Liabilities | $ 14,683 | $ 15,485
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 | 14.93 | 18.51
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 | 37.58 | 38.75
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 | 26.86 | 27.89
Natural gas forward contracts |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Assets | $ 520 | $ 1,015
Liabilities | $ 6,619 | $ 3,807
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 | 1.80 | 2.33
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 | 3.05 | 3.11
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 | 2.51 | 2.71

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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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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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- Definition Fair value of financial and nonfinancial obligations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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