Quarterly report pursuant to Section 13 or 15(d)

Derivative Instruments - Assets and liabilities (Details)

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Derivative Instruments - Assets and liabilities (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Commodity derivative    
Commodity derivative assets    
Gross amounts on balance sheet $ 1,242,069 $ 1,367,554
Gross amounts offset on balance sheet (57,635) (65,612)
Total asset derivatives 1,184,434 1,301,942
Commodity derivative liabilities    
Gross amounts on balance sheet (66,868) (72,901)
Gross amounts offset on balance sheet 57,635 65,612
Total liability derivatives (9,233) (7,289)
Marketing derivative    
Commodity derivative assets    
Gross amounts on balance sheet   311,083
Gross amounts offset on balance sheet   (311,083)
Commodity derivative liabilities    
Gross amounts on balance sheet (21,428) (332,477)
Gross amounts offset on balance sheet   311,083
Total liability derivatives $ (21,428) $ (21,394)