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Derivatives (Details 1) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2017
Derivatives, Fair Value [Line Items]      
Commodity derivative assets $ (18,874) $ 17,252 $ 32,310
Commodity derivative liabilities 0 0 0
Commodity derivative assets - current 43,810 30,702 48,049
Commodity derivative assets - noncurrent 1,739 310 339
Commodity derivative liabilities - current (15,424) (29,651) (29,914)
Commodity derivative liabilities - noncurrent (1,414) (825) (450)
Commodity      
Derivatives, Fair Value [Line Items]      
Commodity derivative assets 34,874 37,730 58,397
Commodity derivative liabilities (60,925) (38,545) (37,604)
Cash collateral 54,762 1,351 (2,769)
Total 28,711 536 18,024
Commodity Derivative Assets - Current | Commodity      
Derivatives, Fair Value [Line Items]      
Commodity derivative assets 29,861 36,929 57,499
Commodity derivative liabilities (40,813) (7,578) (6,681)
Cash collateral 54,762 1,351 (2,769)
Commodity derivative assets - current 43,810 30,702 48,049
Commodity Derivative Assets - Noncurrent | Commodity      
Derivatives, Fair Value [Line Items]      
Commodity derivative assets 1,851 311 341
Commodity derivative liabilities (112) (1) (2)
Cash collateral 0 0 0
Commodity derivative assets - noncurrent 1,739 310 339
Commodity Derivative Liabilities - Current | Commodity      
Derivatives, Fair Value [Line Items]      
Commodity derivative assets 3,115 489 554
Commodity derivative liabilities (18,539) (30,140) (30,468)
Cash collateral 0 0 0
Commodity derivative liabilities - current (15,424) (29,651) (29,914)
Commodity Derivative Liabilities - Noncurrent | Commodity      
Derivatives, Fair Value [Line Items]      
Commodity derivative assets 47 1 3
Commodity derivative liabilities (1,461) (826) (453)
Cash collateral 0 0 0
Commodity derivative liabilities - noncurrent $ (1,414) $ (825) $ (450)