Fair Value (Quantitative Information) (Details) $ in Thousands	Dec. 31, 2018 USD ($) MMBTU $ / MmBtu $ / MWH MWh	Dec. 31, 2017 USD ($)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Liability | $	$ (102,453)	$ (161,467)
Fair Value, Measurements, Recurring [Member] | Power Exchange Agreements [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Liability | $	(2,488)
Fair Value, Measurements, Recurring [Member] | Natural Gas Exchange Agreements [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Liability | $	$ (2,774)
Measurement Input, Operating and Maintenance Charges [Member] | Minimum [Member] | Power Exchange Agreements [Member] | Surrogate Facility Pricing [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Liability Measurement Input | $ / MWH	40.05
Measurement Input, Operating and Maintenance Charges [Member] | Maximum [Member] | Power Exchange Agreements [Member] | Surrogate Facility Pricing [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Liability Measurement Input | $ / MWH	52.59
Measurement Input, Operating and Maintenance Charges [Member] | Weighted Average [Member] | Power Exchange Agreements [Member] | Surrogate Facility Pricing [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Liability Measurement Input | $ / MWH	45.61
Measurement Input, Transaction Volumes [Member] [Member] | Minimum [Member] | Power Exchange Agreements [Member] | Surrogate Facility Pricing [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Liability Measurement Input | MWh	173,465
Measurement Input, Transaction Volumes [Member] [Member] | Maximum [Member] | Power Exchange Agreements [Member] | Surrogate Facility Pricing [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Liability Measurement Input | MWh	173,465
Measurement Input, Transaction Volumes [Member] [Member] | Sales [Member] | Minimum [Member] | Natural Gas Exchange Agreements [Member] | Internally Derived Weighted Average Cost Of Gas [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Liability Measurement Input | MMBTU	60,000
Measurement Input, Transaction Volumes [Member] [Member] | Sales [Member] | Maximum [Member] | Natural Gas Exchange Agreements [Member] | Internally Derived Weighted Average Cost Of Gas [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Liability Measurement Input | MMBTU	310,000
Measurement Input, Transaction Volumes [Member] [Member] | Purchase [Member] | Minimum [Member] | Natural Gas Exchange Agreements [Member] | Internally Derived Weighted Average Cost Of Gas [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Liability Measurement Input | MMBTU	125,000
Measurement Input, Transaction Volumes [Member] [Member] | Purchase [Member] | Maximum [Member] | Natural Gas Exchange Agreements [Member] | Internally Derived Weighted Average Cost Of Gas [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Liability Measurement Input | MMBTU	310,000
Measurement Input, Commodity Forward Price [Member] | Sales [Member] | Minimum [Member] | Natural Gas Exchange Agreements [Member] | Internally Derived Weighted Average Cost Of Gas [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Liability Measurement Input | $ / MmBtu	1.47
Measurement Input, Commodity Forward Price [Member] | Sales [Member] | Maximum [Member] | Natural Gas Exchange Agreements [Member] | Internally Derived Weighted Average Cost Of Gas [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Liability Measurement Input | $ / MmBtu	3.34
Measurement Input, Commodity Forward Price [Member] | Purchase [Member] | Minimum [Member] | Natural Gas Exchange Agreements [Member] | Internally Derived Weighted Average Cost Of Gas [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Liability Measurement Input | $ / MmBtu	1.44
Measurement Input, Commodity Forward Price [Member] | Purchase [Member] | Maximum [Member] | Natural Gas Exchange Agreements [Member] | Internally Derived Weighted Average Cost Of Gas [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Liability Measurement Input | $ / MmBtu	1.88