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Segment Information Reconciliation of Segment Information, Revenue and Other Operating Income (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Revenue, Major Customer [Line Items]        
Gain on Commodity Derivative Instruments $ 25,660 $ 83,788 $ 60,747 $ 61,325
Other Operating Income 8,534 16,658 19,244 32,308
Total Revenue 402,124 371,068 897,853 690,996
Operating Segments [Member]        
Revenue, Major Customer [Line Items]        
Total Segment Revenue from Contracts with External Customers $ 367,930 $ 270,622 $ 817,862 $ 597,363