Fair Value Measurements - Level 3 Quantitative Information (Details) $ in Thousands	12 Months Ended
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

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.

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- Definition

Represents the percentage of implied electricity volatilities.

+ References

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- Definition

Represents the percentage of implied electricity natural gas price volatilities.

+ References

No definition available.

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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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No definition available.

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Fair value of financial and nonfinancial obligations.

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