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Derivative Instruments - Effect of Derivative Instruments on the Condensed Consolidated Balance Sheet (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Effect of derivative instruments on the Consolidated Balance Sheet    
Derivative Assets $ 14,246 $ 57,665
Derivatives Not Designated as Hedges | Commodity contracts    
Effect of derivative instruments on the Consolidated Balance Sheet    
Derivative Assets 14,246 57,665
Derivative Liabilities 1,305 0
Derivative instruments (current) | Derivatives Not Designated as Hedges | Commodity contracts    
Effect of derivative instruments on the Consolidated Balance Sheet    
Derivative Assets 14,246 57,665
Derivative Liabilities $ 1,305 $ 0