Derivatives (Natural Gas Derivative Contracts) (Details) - Natural Gas Derivative Contract	12 Months Ended
	Dec. 31, 2018 MMBTU $ / MMBTU
2019 | Price Swap Contracts
Derivative [Line Items]
Volume in MMbtu | MMBTU	10,905,000
Weighted Average Swap Price (USD per unit)	2.69
2019 | Price Swap Contracts | Maximum
Derivative [Line Items]
Swap Price (USD per unit)	3.09
2019 | Price Swap Contracts | Minimum
Derivative [Line Items]
Swap Price (USD per unit)	2.64
2019 | Short Call Options
Derivative [Line Items]
Volume in MMbtu | MMBTU	4,000,000
Weighted Average Option Price (USD per unit)	3.31
2019 | Short Call Options | Maximum
Derivative [Line Items]
Option Price (USD per unit)	3.75
2019 | Short Call Options | Minimum
Derivative [Line Items]
Option Price (USD per unit)	3.17
2019 | Long Put Options
Derivative [Line Items]
Volume in MMbtu | MMBTU	3,550,000
Weighted Average Option Price (USD per unit)	2.81
2019 | Long Put Options | Maximum
Derivative [Line Items]
Option Price (USD per unit)	2.90
2019 | Long Put Options | Minimum
Derivative [Line Items]
Option Price (USD per unit)	2.70
2019 | Short Put Options
Derivative [Line Items]
Volume in MMbtu | MMBTU	2,425,000
Weighted Average Option Price (USD per unit)	2.27
2019 | Short Put Options | Maximum
Derivative [Line Items]
Option Price (USD per unit)	2.40
2019 | Short Put Options | Minimum
Derivative [Line Items]
Option Price (USD per unit)	2.20
2020 | Short Call Options
Derivative [Line Items]
Volume in MMbtu | MMBTU	2,275,000
Weighted Average Option Price (USD per unit)	3.19
2020 | Short Call Options | Maximum
Derivative [Line Items]
Option Price (USD per unit)	3.20
2020 | Short Call Options | Minimum
Derivative [Line Items]
Option Price (USD per unit)	3.17
2020 | Long Put Options
Derivative [Line Items]
Volume in MMbtu | MMBTU	9,150,000
Weighted Average Option Price (USD per unit)	2.57
2020 | Long Put Options | Maximum
Derivative [Line Items]
Option Price (USD per unit)	2.70
2020 | Long Put Options | Minimum
Derivative [Line Items]
Option Price (USD per unit)	2.50
2020 | Short Put Options
Derivative [Line Items]
Volume in MMbtu | MMBTU	9,150,000
Weighted Average Option Price (USD per unit)	2.07
2020 | Short Put Options | Maximum
Derivative [Line Items]
Option Price (USD per unit)	2.20
2020 | Short Put Options | Minimum
Derivative [Line Items]
Option Price (USD per unit)	2.00
2021 | Long Put Options
Derivative [Line Items]
Volume in MMbtu | MMBTU	2,250,000
Weighted Average Option Price (USD per unit)	2.65
2021 | Long Put Options | Maximum
Derivative [Line Items]
Option Price (USD per unit)	2.65
2021 | Long Put Options | Minimum
Derivative [Line Items]
Option Price (USD per unit)	2.65
2021 | Short Put Options
Derivative [Line Items]
Volume in MMbtu | MMBTU	2,250,000
Weighted Average Option Price (USD per unit)	2.15
2021 | Short Put Options | Maximum
Derivative [Line Items]
Option Price (USD per unit)	2.15
2021 | Short Put Options | Minimum
Derivative [Line Items]
Option Price (USD per unit)	2.15