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Derivatives - Fair Value of Derivative Contracts (Details) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Derivatives, Fair Value    
Derivative assets $ 1,133 $ 5,286
Total net derivative contracts 1,133 5,286
Current assets    
Derivatives, Fair Value    
Derivative assets 1,133 5,286
Oil price swaps | Current assets    
Derivatives, Fair Value    
Derivative assets 1,133 0
Natural gas price swaps | Current assets    
Derivatives, Fair Value    
Derivative assets $ 0 $ 5,286