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Estimated quantities of proved reserves (Details 5) | 12 Months Ended
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Dec. 31, 2015 MBoe MMcf MBbls | Dec. 31, 2014 MBoe MMcf MBbls | Dec. 31, 2013 MBoe MMcf MBbls
Proved developed and undeveloped reserves: |  |  |
Balance at beginning of year, BOE | MBoe | 91,940 | 80,695 | 80,486
Production, BOE | MBoe | (6,471) | (8,998) | (10,264)
Extensions and discoveries, BOE | MBoe | 888 | 24,025 | 19,050
Improved recovery, BOE | MBoe | 0 | 0 | 0
Purchases of proved reserves, BOE | MBoe | 0 | 6,748 | 0
Sales revisions, BOE | MBoe | (57,097) | (14,796) | (7,987)
Revisions of previous estimate, BOE | MBoe | (15,939) | 4,266 | (590)
Balance at end of year, BOE | MBoe | 13,321 | 91,940 | 80,695
Oil (MBbls) |  |  |
Proved developed and undeveloped reserves: |  |  |
Balance at beginning of year | 43,918 | 41,019 | 33,453
Production | (3,286) | (4,919) | (4,815)
Extensions and discoveries | 744 | 9,654 | 13,313
Improved recovery | 0 | 0 | 0
Purchases of proved reserves | 0 | 5,463 | 0
Sales of proved reserves | (16,474) | (4,945) | (1,286)
Revisions of previous estimates | (12,215) | (2,354) | 354
Balance at end of year | 12,687 | 43,918 | 41,019
NGL (MBbls) |  |  |
Proved developed and undeveloped reserves: |  |  |
Balance at beginning of year | 7,187 | 6,602 | 7,153
Production | (393) | (609) | (781)
Extensions and discoveries | 29 | 3,634 | 1,333
Improved recovery | 0 | 0 | 0
Purchases of proved reserves | 0 | 0 | 0
Sales of proved reserves | (6,864) | (3,109) | (25)
Revisions of previous estimates | 252 | 669 | (1,078)
Balance at end of year | 211 | 7,187 | 6,602
Natural Gas (MMcf) |  |  |
Proved developed and undeveloped reserves: |  |  |
Balance at beginning of year | MMcf | 245,011 | 198,445 | 239,278
Production | MMcf | (16,747) | (20,822) | (28,008)
Extensions and discoveries | MMcf | 681 | 64,420 | 26,428
Improved recovery | MMcf | 0 | 0 | 0
Purchases of proved reserves | MMcf | 0 | 7,711 | 0
Sales of proved reserves | MMcf | (202,560) | (40,451) | (40,055)
Revisions of previous estimates | MMcf | (23,854) | 35,708 | 802
Balance at end of year | MMcf | 2,531 | 245,011 | 198,445

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- Definition Energy measure of increase in quantity of proved reserve of oil and gas resulting from the extension of the proved acreage of previously discovered reservoir or discovery of new field with proved reserve or of new reservoir of proved reserve in old field, stated in a unit of energy. For example, but not limited to, barrel of oil equivalents (BOE) or one thousand cubic feet of natural gas equivalent (Mcfe). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 5 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=51678428&loc=d3e61831-109447 + Details
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- Definition Energy measure of increase in quantity of proved reserve of oil and gas resulting from application of improved recovery techniques, stated in a unit of energy. For example, but not limited to, barrel of oil equivalents (BOE) or one thousand cubic feet of natural gas equivalent (Mcfe). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 5 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=51678428&loc=d3e61831-109447 + Details
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- Definition Energy measure of decrease in quantity of proved reserve of oil and gas resulting from production, stated in a unit of energy. For example, but not limited to, barrel of oil equivalents (BOE) or one thousand cubic feet of natural gas equivalent (Mcfe). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 5 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=51678428&loc=d3e61831-109447 + Details
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- Definition Additions to proved reserves that result from (1) extension of the proved acreage of previously discovered (old) reservoirs through additional drilling in periods after discovery and (2) discovery of new fields with proved reserves or of new reservoirs of proved reserves in old fields. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 5 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=51678428&loc=d3e61831-109447 + Details
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- Definition Changes in proved reserve estimates resulting from application of improved recovery techniques. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 5 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=51678428&loc=d3e61831-109447 + Details
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- Definition The net quantity of proved reserves as of the balance sheet date. Proved oil reserves are the estimated quantities of crude oil and natural gas liquids which geological and engineering data demonstrate with reasonable certainty to be recoverable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=51678428&loc=d3e61831-109447 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-10.(a)(2)) -URI http://asc.fasb.org/extlink&oid=50486907&loc=d3e511914-122862 + Details
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