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Derivatives (Details 1) - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
Jun. 30, 2016
Derivatives, Fair Value [Line Items]      
Commodity derivative assets $ (12,835) $ 1,599 $ 13,491
Commodity derivative liabilities 0 0 (1,480)
Commodity derivative assets - current 11,619 45,447 115,924
Commodity derivative assets - noncurrent 1,191 100 1,934
Commodity derivative liabilities - current (18,104) (23,167) (43,183)
Commodity derivative liabilities - noncurrent (334) (339) (2,182)
Commodity      
Derivatives, Fair Value [Line Items]      
Commodity derivative assets 31,708 37,739 142,608
Commodity derivative liabilities (52,788) (43,971) (107,887)
Cash collateral 15,452 28,273 37,772
Total (5,628) 22,041 72,493
Commodity Derivative Assets - Current | Commodity      
Derivatives, Fair Value [Line Items]      
Commodity derivative assets 26,101 36,146 134,504
Commodity derivative liabilities (29,934) (18,972) (56,832)
Cash collateral 15,452 28,273 38,252
Commodity derivative assets - current 11,619 45,447 115,924
Commodity Derivative Assets - Noncurrent | Commodity      
Derivatives, Fair Value [Line Items]      
Commodity derivative assets 1,201 140 2,095
Commodity derivative liabilities (10) (40) (161)
Cash collateral 0 0 0
Commodity derivative assets - noncurrent 1,191 100 1,934
Commodity Derivative Liabilities - Current | Commodity      
Derivatives, Fair Value [Line Items]      
Commodity derivative assets 4,404 1,447 5,925
Commodity derivative liabilities (22,508) (24,614) (48,628)
Cash collateral 0 0 (480)
Commodity derivative liabilities - current (18,104) (23,167) (43,183)
Commodity Derivative Liabilities - Noncurrent | Commodity      
Derivatives, Fair Value [Line Items]      
Commodity derivative assets 2 6 84
Commodity derivative liabilities (336) (345) (2,266)
Cash collateral 0 0 0
Commodity derivative liabilities - noncurrent $ (334) $ (339) $ (2,182)