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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, 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 | $ 23,847 | $ 30,364
Liabilities | 63,354 | 63,343
Electricity forward contracts |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Assets | 21,606 | 24,543
Liabilities | $ 51,305 | $ 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 | 13.97 | 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 | 39.61 | 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 | 26.45 | 26.86
Option Contracts |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Assets | $ 0 | $ 0
Liabilities | $ 5,880 | $ 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 | 19.46 | 23.87
Electricity price volatilities (as a percent) | 47.00% | 40.00%
Natural gas price volatilities (as a percent) | 38.00% | 32.00%
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 | 37.93 | 44.13
Electricity price volatilities (as a percent) | 72.00% | 59.00%
Natural gas price volatilities (as a percent) | 45.00% | 40.00%
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 | 29.39 | 33.91
Electricity price volatilities (as a percent) | 63.00% | 52.00%
Natural gas price volatilities (as a percent) | 41.00% | 35.00%
Natural gas forward contracts |  |
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments |  |
Assets | $ 2,241 | $ 5,821
Liabilities | $ 6,169 | $ 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 | 1.65 | 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 | 3.11 | 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 | 2.67 | 2.61

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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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