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Derivative Instruments Fair Value of Derivative Instruments (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Commodity Swaps [Member]    
Asset Derivative Instruments    
Prepaid Expense $ 27,226 $ 62,369
Other Assets 132,594 59,281
Total Asset 159,820 121,650
Liability Derivative Instruments    
Other Accrued Liabilities 26,943 5,985
Other Liabilities 10,381 42,419
Total Liability 37,324 48,404
Basis Swap [Member]    
Asset Derivative Instruments    
Prepaid Expense 10,932 14,965
Other Assets 35,101 24,223
Total Asset 46,033 39,188
Liability Derivative Instruments    
Other Accrued Liabilities 26,969 35,306
Other Liabilities 20,557 17,179
Total Liability $ 47,526 $ 52,485