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Derivatives - Fair Value of Derivative Contracts (Details)
$ in Thousands
Dec. 31, 2018
USD ($)
Derivatives, Fair Value  
Derivative assets $ 5,286
Total net derivative contracts (5,286)
Current assets  
Derivatives, Fair Value  
Derivative assets 5,286
Natural gas price swaps | Current assets  
Derivatives, Fair Value  
Derivative assets $ 5,286