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Subject to master netting agreements (Details 4) - USD ($) $ in Thousands | Jun. 30, 2015 | Dec. 31, 2014 | Jun. 30, 2014
--- | --- | --- | ---
Commodity derivative instruments assets [Member] |  |  |
Derivative instruments [Line Items] |  |  |
Derivative asset, fair value, gross asset | $ 2,537 | $ 18,335 | $ 260
Gross amount not offset | (2,537) | 0 | (260)
Derivative asset, fair value, net | 0 | 18,335 | 0
Total assets [Member] |  |  |
Derivative instruments [Line Items] |  |  |
Derivative asset, fair value, gross asset | 2,537 | 18,335 | 260
Gross amount not offset | (2,537) | 0 | (260)
Derivative asset, fair value, net | 0 | $ 18,335 | 0
Commodity derivatives - liabilities [Member] |  |  |
Derivative instruments [Line Items] |  |  |
Derivative liability fair value gross liability | 3,511 |  | 17,449
Gross amount not offset | (2,537) |  | (260)
Derivative liability, fair value, net | 974 |  | 17,189
Total liabilities [Member] |  |  |
Derivative instruments [Line Items] |  |  |
Derivative liability fair value gross liability | 3,511 |  | 17,449
Gross amount not offset | (2,537) |  | (260)
Derivative liability, fair value, net | $ 974 |  | $ 17,189

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