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Estimated quantities of proved reserves (Details 4)
 12 Months Ended
Dec. 31, 2013
MBoe
Dec. 31, 2012
MBoe
Dec. 31, 2011
MBoe
Proved developed and undeveloped reserves:      
Balance at beginning of year, BOE 80,486 97,651 107,599
Production, BOE (10,264) (10,058) (11,099)
Extensions and discoveries, BOE 19,050 14,756 10,842
Improved recovery, BOE 0 0 0
Purchases of proved reserves, BOE 0 0 247
Sales revisions, BOE (7,987) (423) 0
Revisions of previous estimate, BOE (590) (21,440) (9,938)
Balance at end of year, BOE 80,695 80,486 97,651
Reduction in proved undeveolped reserves specified properties     8,900
Oil (MBbls)
      
Proved developed and undeveloped reserves:      
Balance at beginning of year 33,453 27,005 25,666
Production (4,815) (3,694) (2,724)
Extensions and discoveries 13,313 9,874 4,717
Improved recovery 0 0 0
Purchases of proved reserves 0 0 223
Sales of proved reserves (1,286) (39) 0
Revisions of previous estimates 354 307 (877)
Balance at end of year 41,019 33,453 27,005
NGL (MBbls)
      
Proved developed and undeveloped reserves:      
Balance at beginning of year 7,153 7,342 7,201
Production (781) (828) (776)
Extensions and discoveries 1,333 1,817 1,421
Improved recovery 0 0 0
Purchases of proved reserves 0 0 16
Sales of proved reserves (25) 0 0
Revisions of previous estimates (1,078) (1,178) (520)
Balance at end of year 6,602 7,153 7,342
Natural Gas (MMcf)
      
Proved developed and undeveloped reserves:      
Balance at beginning of year 239,278 379,827 448,397
Production (28,008) (33,214) (45,598)
Extensions and discoveries 26,428 18,386 28,221
Improved recovery 0 0 0
Purchases of proved reserves 0 0 54
Sales of proved reserves (40,055) (2,307) 0
Revisions of previous estimates 802 (123,414) (51,247)
Balance at end of year 198,445 239,278 379,827