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Subject to master netting agreements (Details 4) (USD $) In Thousands, unless otherwise specified | Jun. 30, 2013 | Dec. 31, 2012 | Jun. 30, 2012
--- | --- | --- | ---
Derivative instruments [Line Items] |  |  |
Derivative asset, fair value, gross asset | $ 11,244 | $ 18,304 | $ 48,657
Derivative liability fair value gross liability | 1,388 | 6,255 | 8,000
Commodity derivatives [Member] |  |  |
Derivative instruments [Line Items] |  |  |
Derivative asset, fair value, gross asset | 11,244 | 18,304 | 48,657
Gross amount not offset | (1,388) | 0 | (809)
Derivative asset, fair value, net | 9,856 | 18,304 | 47,848
Total assets [Member] |  |  |
Derivative instruments [Line Items] |  |  |
Derivative asset, fair value, gross asset | 11,244 | 18,304 | 48,657
Gross amount not offset | (1,388) | 0 | (809)
Derivative asset, fair value, net | 9,856 | 18,304 | 47,848
Commodity derivatives - liabilities [Member] |  |  |
Derivative instruments [Line Items] |  |  |
Derivative liability fair value gross liability | 1,388 |  | 1,037
Gross amount not offset | (1,388) |  | (809)
Derivative liability, fair value, net | 0 |  | 228
Interest rate derivatives - liabilities [Member] |  |  |
Derivative instruments [Line Items] |  |  |
Derivative liability fair value gross liability |  | 6,255 | 6,963
Gross amount not offset |  | 0 | 0
Derivative liability, fair value, net |  | 6,255 | 6,963
Total liabilities [Member] |  |  |
Derivative instruments [Line Items] |  |  |
Derivative liability fair value gross liability | 1,388 | 6,255 | 8,000
Gross amount not offset | (1,388) | 0 | (809)
Derivative liability, fair value, net | $ 0 | $ 6,255 | $ 7,191

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