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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

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