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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 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Derivative [Line Items]        
Net settlements $ 9,585 $ 47,728 $ 22,151 $ 167,859
Net change in fair value of unsettled derivatives (61,763) (28,331) 64,307 (230,207)
Total commodity price risk management gain (loss), net $ (52,178) $ 19,397 $ 86,458 $ (62,348)