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Derivative Instruments Fair Value of Derivative Instruments (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Commodity Swaps [Member]    
Asset Derivative Instruments    
Prepaid Expense $ 46,668 $ 62,369
Other Assets 80,655 59,281
Total Asset 127,323 121,650
Liability Derivative Instruments    
Other Accrued Liabilities 20,908 5,985
Other Liabilities 30,369 42,419
Total Liability 51,277 48,404
Basis Swap [Member]    
Asset Derivative Instruments    
Prepaid Expense 25,922 14,965
Other Assets 43,481 24,223
Total Asset 69,403 39,188
Liability Derivative Instruments    
Other Accrued Liabilities 17,864 35,306
Other Liabilities 15,557 17,179
Total Liability $ 33,421 $ 52,485