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Fair Value of Financial Instruments Financial Instruments Measured at Fair Value on a Recurring Basis (Details) - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Gas Derivatives $ 0 $ 0
Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Gas Derivatives 405,781 99,456
Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Gas Derivatives $ 0 $ 0