Fair Value Measurements - Significant Unobservable Inputs Used to Value Level 3 Instruments (Details 2) $ in Thousands	Jun. 30, 2018 USD ($) $ / MWh $ / MMBTU	Dec. 31, 2017 USD ($) $ / MWh $ / MMBTU
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Liabilities	$ 95,443	$ 96,422
Significant Unobservable Inputs (Level 3)
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Assets	2,071	1,036
Liabilities	11,429	19,292
Significant Unobservable Inputs (Level 3) \| Electricity forward contracts
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Assets	0	21
Liabilities	1,928	15,485
Significant Unobservable Inputs (Level 3) \| Option Contracts
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Assets	1,410
Liabilities	203
Significant Unobservable Inputs (Level 3) \| Natural gas contracts
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Assets	661	1,015
Liabilities	$ 9,298	$ 3,807
Discounted cash flows \| Forward Price \| Electricity forward contracts \| Weighted Average
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Significant Unobservable Input \| $ / MWh		27.89
Discounted cash flows \| Forward Price \| Natural gas contracts \| Weighted Average
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Significant Unobservable Input \| $ / MWh		2.71
Discounted cash flows \| Significant Unobservable Inputs (Level 3) \| Forward Price \| Electricity forward contracts \| Minimum
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Significant Unobservable Input \| $ / MWh	20.43	18.51
Discounted cash flows \| Significant Unobservable Inputs (Level 3) \| Forward Price \| Electricity forward contracts \| Maximum
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Significant Unobservable Input \| $ / MWh	63.14	38.75
Discounted cash flows \| Significant Unobservable Inputs (Level 3) \| Forward Price \| Electricity forward contracts \| Weighted Average
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Significant Unobservable Input \| $ / MWh	60.68
Discounted cash flows \| Significant Unobservable Inputs (Level 3) \| Forward Price \| Natural gas contracts \| Minimum
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Significant Unobservable Input \| $ / MMBTU	1.65	2.33
Discounted cash flows \| Significant Unobservable Inputs (Level 3) \| Forward Price \| Natural gas contracts \| Maximum
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Significant Unobservable Input \| $ / MMBTU	2.82	3.11
Discounted cash flows \| Significant Unobservable Inputs (Level 3) \| Forward Price \| Natural gas contracts \| Weighted Average
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Significant Unobservable Input \| $ / MWh	2.32
Option model \| Significant Unobservable Inputs (Level 3) \| Forward Price \| Option Contracts \| Minimum
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Significant Unobservable Input	0.93
Option model \| Significant Unobservable Inputs (Level 3) \| Forward Price \| Option Contracts \| Maximum
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Significant Unobservable Input	1.10
Option model \| Significant Unobservable Inputs (Level 3) \| Electricity price volatilities \| Option Contracts \| Weighted Average
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Significant Unobservable Input	1.01

Value of input used to measure net derivative asset (liability).

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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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