Quarterly report pursuant to Section 13 or 15(d)

Derivative Financial Instruments - Summary of Open and Closed (Not Settled) Commodity Derivative Contracts (Details)

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Derivative Financial Instruments - Summary of Open and Closed (Not Settled) Commodity Derivative Contracts (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Derivatives Fair Value [Line Items]    
Total contracts $ 5,281 $ 84
Open Contracts    
Derivatives Fair Value [Line Items]    
Total contracts 5,070  
Closed Contracts - Not Settled    
Derivatives Fair Value [Line Items]    
Total contracts $ 211 $ 84