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Derivatives (Details 1) (USD $)
In Thousands, unless otherwise specified
Jun. 30, 2014
Dec. 31, 2013
Jun. 30, 2013
Derivatives, Fair Value [Line Items]      
Commodity derivative assets $ 52,731 $ 31,055 $ 46,329
Commodity derivative liabilities 0 0 0
Commodity derivative assets - current 162,427 71,319 121,789
Commodity derivative assets - noncurrent 312 246 87
Commodity derivative liabilities - current (86,134) (63,954) (58,183)
Commodity derivative liabilities - noncurrent (7,444) (6,644) (5,863)
Commodity
     
Derivatives, Fair Value [Line Items]      
Commodity derivative assets 116,931 70,870 103,184
Commodity derivative liabilities (109,762) (85,383) (78,718)
Cash collateral 61,992 15,480 33,364
Total 69,161 967 57,830
Commodity | Commodity derivative assets - current
     
Derivatives, Fair Value [Line Items]      
Commodity derivative assets 113,205 69,289 98,902
Commodity derivative liabilities (12,770) (13,450) (10,477)
Cash collateral 61,992 15,480 33,364
Commodity derivative assets - current 162,427 71,319 121,789
Commodity | Commodity derivative assets - noncurrent
     
Derivatives, Fair Value [Line Items]      
Commodity derivative assets 323 246 87
Commodity derivative liabilities (11) 0 0
Cash collateral 0 0 0
Commodity derivative assets - noncurrent 312 246 87
Commodity | Commodity derivative liabilities - current
     
Derivatives, Fair Value [Line Items]      
Commodity derivative assets 3,143 1,286 4,133
Commodity derivative liabilities (89,277) (65,240) (62,316)
Cash collateral 0 0 0
Commodity derivative liabilities - current (86,134) (63,954) (58,183)
Commodity | Commodity derivative liabilities - noncurrent
     
Derivatives, Fair Value [Line Items]      
Commodity derivative assets 260 49 62
Commodity derivative liabilities (7,704) (6,693) (5,925)
Cash collateral 0 0 0
Commodity derivative liabilities - noncurrent $ (7,444) $ (6,644) $ (5,863)