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Change in discounted future net cash flows (Details 8) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013
Change in the standardized measure of discounted future net cash flows [Roll Forward] |  |
Beginning of year | $ 1,014,000 | $ 883,400
Net revenues from production | (368,900) | (398,000)
Net change in sales prices and production costs related to future production | 86,300 | 162,200
Extensions and discoveries, net of future production-related costs | 231,900 | 366,500
Improved recovery, net of future production-related costs | 0 | 0
Purchases of proved reserves, net of future production-related costs | 103,800 | 0
Sales of proved reserves | (219,300) | (37,800)
Changes in estimated future development costs | 65,100 | 6,700
Development costs incurred during the current year | 104,600 | 141,500
Accretion of discount | 109,400 | 94,600
Net change in income taxes | (33,400) | (141,400)
Revisions of previous estimates | (16,300) | (55,800)
Other | (2,300) | (7,900)
Net change | 60,900 | 130,600
End of year | $ 1,074,900 | $ 1,014,000

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