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Financial Instruments Measured at Fair Value on a Recurring Basis (Details) - USD ($)
$ in Thousands
Mar. 31, 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 112,028 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