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Subject to master netting agreements (Details 4) (USD $)
In Thousands, unless otherwise specified
Sep. 30, 2013
Dec. 31, 2012
Sep. 30, 2012
Derivative instruments [Line Items]      
Derivative asset, fair value, gross asset $ 6,144 $ 18,304 $ 22,719
Derivative liability fair value gross liability 9,889 6,255 9,934
Commodity derivatives [Member]
      
Derivative instruments [Line Items]      
Derivative asset, fair value, gross asset 6,144 18,304 22,719
Gross amount not offset (4,939) 0 (2,102)
Derivative asset, fair value, net 1,205 18,304 20,617
Total assets [Member]
      
Derivative instruments [Line Items]      
Derivative asset, fair value, gross asset 6,144 18,304 22,719
Gross amount not offset (4,939) 0 (2,102)
Derivative asset, fair value, net 1,205 18,304 20,617
Commodity derivatives - liabilities [Member]
      
Derivative instruments [Line Items]      
Derivative liability fair value gross liability 9,889   2,155
Gross amount not offset (4,939)   (2,102)
Derivative liability, fair value, net 4,950   53
Interest rate derivatives - liabilities [Member]
      
Derivative instruments [Line Items]      
Derivative liability fair value gross liability   6,255 7,779
Gross amount not offset   0 0
Derivative liability, fair value, net   6,255 7,779
Total liabilities [Member]
      
Derivative instruments [Line Items]      
Derivative liability fair value gross liability 9,889 6,255 9,934
Gross amount not offset (4,939) 0 (2,102)
Derivative liability, fair value, net $ 4,950 $ 6,255 $ 7,832