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Proved reserves (Details 6) | Dec. 31, 2015 MBoe MMcf MBbls | Dec. 31, 2014 MBoe MMcf MBbls | Dec. 31, 2013 MBoe MMcf MBbls | Dec. 31, 2012 MBoe MMcf MBbls
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Proved Developed and Undeveloped Reserves [Abstract] |  |  |  |
Proved developed reserves, energy | MBoe | 11,865 | 65,086 | 66,140 |
Proved undeveloped reserves, energy | MBoe | 1,456 | 26,854 | 14,555 |
Balance at end of year, energy | MBoe | 13,321 | 91,940 | 80,695 | 80,486
Changes in proved and undeveloped reserves [Abstract] |  |  |  |
Change in PUD reserve | MBoe | (25,400) |  |  |
Oil (MBbls) |  |  |  |
Proved Developed and Undeveloped Reserves [Abstract] |  |  |  |
Proved developed reserves, volume | 11,380 | 30,130 | 31,394 |
Proved undeveloped reserves, volume | 1,307 | 13,788 | 9,625 |
Balance at end of year, volume | 12,687 | 43,918 | 41,019 | 33,453
NGL (MBbls) |  |  |  |
Proved Developed and Undeveloped Reserves [Abstract] |  |  |  |
Proved developed reserves, volume | 144 | 4,217 | 5,322 |
Proved undeveloped reserves, volume | 67 | 2,970 | 1,280 |
Balance at end of year, volume | 211 | 7,187 | 6,602 | 7,153
Natural Gas (MMcf) |  |  |  |
Proved Developed and Undeveloped Reserves [Abstract] |  |  |  |
Proved developed reserves, volume | MMcf | 2,033 | 184,437 | 176,546 |
Proved undeveloped reserves, volume | MMcf | 498 | 60,574 | 21,899 |
Balance at end of year, volume | MMcf | 2,531 | 245,011 | 198,445 | 239,278

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- Definition Energy measure of proved reserve of oil and gas, 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 -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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- Definition Energy measure of interests in proved developed reserves of crude oil, including condensate and natural gas liquids, natural gas, synthetic oil and gas, or other nonrenewable natural resource that is intended to be upgraded into synthetic oil and gas. + References No definition available. + Details
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- Definition Net quantities of an enterprise's interests in proved developed reserves of either crude oil (including condensate and natural gas liquids), natural gas, synthetic oil and gas, or other nonrenewable natural resource that is intended to be upgraded into synthetic oil and gas as of the beginning and the end of the year. "Net" quantities of reserves include those relating to the enterprise's operating and nonoperating interests in properties. Quantities of reserves relating to royalty interests owned are included in "net" quantities if the necessary information is available to the enterprise. "Net" quantities does not include reserves relating to interests of others in properties owned by the enterprise. The unit of measure for reserve quantities is defined as "barrels" for oil and synthetic oil reserves or "cubic feet" or "cubic meters" for natural gas and synthetic gas reserves. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 9 -URI http://asc.fasb.org/extlink&oid=51678428&loc=d3e61884-109447 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=51678428&loc=d3e61797-109447 + Details
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- Definition Net quantities of an enterprise's interests in proved undeveloped reserves of either crude oil (including condensate and natural gas liquids), natural gas, synthetic oil and gas, or other renewable natural resource that is intended to be upgraded into synthetic oil and gas as of the beginning and the end of the year. "Net" quantities of reserves include those relating to the enterprise's operating and nonoperating interests in properties. Quantities of reserves relating to royalty interests owned are included in "net" quantities if the necessary information is available to the enterprise. "Net" quantities does not include reserves relating to interests of others in properties owned by the enterprise. The unit of measure for reserve quantities is defined as "barrels" for oil and synthetic oil reserves or "cubic feet" or "cubic meters" for natural gas and synthetic gas reserves. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 9 -URI http://asc.fasb.org/extlink&oid=51678428&loc=d3e61884-109447 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=51678428&loc=d3e61797-109447 + Details
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