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Fair Value Measurements - Classification of Outstanding Derivative Instruments and Fair Value Hierarchy (Details) - Fair Value, Recurring - USD ($)
$ in Thousands
Jun. 30, 2021
Dec. 31, 2020
Current assets:    
Natural gas derivative instruments $ 0 $ 1,375
Natural gas derivative instruments, Netting 0 (1,098)
Natural gas derivative instruments, Carrying Amount 0 277
Current liabilities:    
Natural gas derivative instruments 2,043 1,098
Natural gas derivative instruments, Netting 0 (1,098)
Natural gas derivative instruments, Carrying Amount 2,043 0
Level 1    
Current assets:    
Natural gas derivative instruments 0 0
Current liabilities:    
Natural gas derivative instruments 0 0
Level 2    
Current assets:    
Natural gas derivative instruments 0 1,375
Current liabilities:    
Natural gas derivative instruments 2,043 1,098
Level 3    
Current assets:    
Natural gas derivative instruments 0 0
Current liabilities:    
Natural gas derivative instruments $ 0 $ 0