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Fair Value Measurements - Level 3 Quantitative Information (Details) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 USD ($) $ / MMBTU $ / MWh | Dec. 31, 2016 USD ($) $ / MMBTU $ / MWh
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Assets | $ 1,036 | $ 11,076
Liabilities | 19,292 | 58,482
Electricity forward contracts |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Assets | 21 | 10,648
Liabilities | $ 15,485 | $ 32,042
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 | 18.51 | 16.43
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 | 38.75 | 41.07
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 | 27.89 | 29.86
Natural gas forward contracts |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Assets | $ 1,015 | $ 428
Liabilities | $ 3,807 | $ 26,440
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 | 2.33 | 2.32
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 | 3.11 | 3.60
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 |  |
Electricity forward price (per MWh) | $ / MWh | 2.71 |
Natural gas forward price (per MMbtu) | $ / MMBTU |  | 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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- 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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