Fair Value (Quantitative Information) (Details) $ in Thousands	Dec. 31, 2019 USD ($) MMBTU $ / MmBtu	Dec. 31, 2018 USD ($)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Liability | $	$ (82,203)	$ (102,453)
Fair Value, Recurring [Member] | Natural Gas Exchange Agreements [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Liability | $	$ (2,976)
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	130,000
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	50,000
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	1.60
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	3.80
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	1.49
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	2.38