Financial Instruments Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]
Gas Derivatives	$ 0	$ 0
Put Option	0	0
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]
Gas Derivatives	136,184	59,949
Put Option	0	(3,500)
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]
Gas Derivatives	0	0
Put Option	$ 0	$ 0