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Impact of Derivative Instruments on Statement of Operations (Details) - Commodity Price Risk Management (loss), net - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Derivative [Line Items]        
Net settlements $ (16,408) $ 12,015 $ (42,446) $ 12,566
Net change in fair value of unsettled derivatives (99,718) 45,917 (120,920) 126,070
Total commodity price risk management gain (loss), net $ (116,126) $ 57,932 $ (163,366) $ 138,636