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Fair Value of Financial Instruments - Financial Instruments Measured at Fair Value on a Recurring Basis (Details) - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Fair Value, Inputs, Level 1    
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]    
Gas Derivatives $ 0 $ 0
Fair Value, Inputs, Level 2    
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]    
Gas Derivatives 156,372 99,456
Fair Value, Inputs, Level 3    
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]    
Gas Derivatives $ 0 $ 0