Fair Value Measurements - Significant Unobservable Inputs Used to Value Level 3 Instruments (Details 2) $ in Thousands	3 Months Ended	12 Months Ended
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

Represents the electricity forward price per Megawatt Hours, used as an input to measure fair value.

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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

Represents the percentage of implied electricity volatilities.

+ References

No definition available.

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- Definition

Represents the percentage of implied electricity natural gas price volatilities.

+ 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

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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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