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Estimated quantities of proved reserves (Details 4) | 12 Months Ended
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Dec. 31, 2014 MBoe | Dec. 31, 2013 MBoe | Dec. 31, 2012 MBoe
Proved developed and undeveloped reserves: |  |  |
Balance at beginning of year, BOE | 80,695 us-gaap_ProvedDevelopedAndUndevelopedReserveNetEnergy | 80,486 us-gaap_ProvedDevelopedAndUndevelopedReserveNetEnergy | 97,651 us-gaap_ProvedDevelopedAndUndevelopedReserveNetEnergy
Production, BOE | (8,998)us-gaap_ProvedDevelopedAndUndevelopedReserveProductionEnergy | (10,264)us-gaap_ProvedDevelopedAndUndevelopedReserveProductionEnergy | (10,058)us-gaap_ProvedDevelopedAndUndevelopedReserveProductionEnergy
Extensions and discoveries, BOE | 24,025 us-gaap_ProvedDevelopedAndUndevelopedReserveExtensionAndDiscoveryEnergy | 19,050 us-gaap_ProvedDevelopedAndUndevelopedReserveExtensionAndDiscoveryEnergy | 14,756 us-gaap_ProvedDevelopedAndUndevelopedReserveExtensionAndDiscoveryEnergy
Improved recovery, BOE | 0 us-gaap_ProvedDevelopedAndUndevelopedReserveImprovedRecoveryEnergy | 0 us-gaap_ProvedDevelopedAndUndevelopedReserveImprovedRecoveryEnergy | 0 us-gaap_ProvedDevelopedAndUndevelopedReserveImprovedRecoveryEnergy
Purchases of proved reserves, BOE | 6,748 us-gaap_ProvedDevelopedAndUndevelopedReservePurchaseOfMineralInPlaceEnergy | 0 us-gaap_ProvedDevelopedAndUndevelopedReservePurchaseOfMineralInPlaceEnergy | 0 us-gaap_ProvedDevelopedAndUndevelopedReservePurchaseOfMineralInPlaceEnergy
Sales revisions, BOE | (14,796)us-gaap_ProvedDevelopedAndUndevelopedReservesSaleOfMineralInPlaceEnergy | (7,987)us-gaap_ProvedDevelopedAndUndevelopedReservesSaleOfMineralInPlaceEnergy | (423)us-gaap_ProvedDevelopedAndUndevelopedReservesSaleOfMineralInPlaceEnergy
Revisions of previous estimate, BOE | 4,266 us-gaap_ProvedDevelopedAndUndevelopedReserveRevisionOfPreviousEstimateEnergy | (590)us-gaap_ProvedDevelopedAndUndevelopedReserveRevisionOfPreviousEstimateEnergy | (21,440)us-gaap_ProvedDevelopedAndUndevelopedReserveRevisionOfPreviousEstimateEnergy
Balance at end of year, BOE | 91,940 us-gaap_ProvedDevelopedAndUndevelopedReserveNetEnergy | 80,695 us-gaap_ProvedDevelopedAndUndevelopedReserveNetEnergy | 80,486 us-gaap_ProvedDevelopedAndUndevelopedReserveNetEnergy
Oil (MBbls) |  |  |
Proved developed and undeveloped reserves: |  |  |
Balance at beginning of year | 41,019 us-gaap_ProvedDevelopedAndUndevelopedReservesNet / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_OilReservesMember | 33,453 us-gaap_ProvedDevelopedAndUndevelopedReservesNet / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_OilReservesMember | 27,005 us-gaap_ProvedDevelopedAndUndevelopedReservesNet / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_OilReservesMember
Production | (4,919)us-gaap_ProvedDevelopedAndUndevelopedReservesProduction / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_OilReservesMember | (4,815)us-gaap_ProvedDevelopedAndUndevelopedReservesProduction / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_OilReservesMember | (3,694)us-gaap_ProvedDevelopedAndUndevelopedReservesProduction / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_OilReservesMember
Extensions and discoveries | 9,654 us-gaap_ProvedDevelopedAndUndevelopedReservesExtensionsDiscoveriesAndAdditions / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_OilReservesMember | 13,313 us-gaap_ProvedDevelopedAndUndevelopedReservesExtensionsDiscoveriesAndAdditions / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_OilReservesMember | 9,874 us-gaap_ProvedDevelopedAndUndevelopedReservesExtensionsDiscoveriesAndAdditions / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_OilReservesMember
Improved recovery | 0 us-gaap_ProvedDevelopedAndUndevelopedReservesImprovedRecovery / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_OilReservesMember | 0 us-gaap_ProvedDevelopedAndUndevelopedReservesImprovedRecovery / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_OilReservesMember | 0 us-gaap_ProvedDevelopedAndUndevelopedReservesImprovedRecovery / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_OilReservesMember
Purchases of proved reserves | 5,463 us-gaap_ProvedDevelopedAndUndevelopedReservesPurchasesOfMineralsInPlace / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_OilReservesMember | 0 us-gaap_ProvedDevelopedAndUndevelopedReservesPurchasesOfMineralsInPlace / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_OilReservesMember | 0 us-gaap_ProvedDevelopedAndUndevelopedReservesPurchasesOfMineralsInPlace / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_OilReservesMember
Sales of proved reserves | (4,945)us-gaap_ProvedDevelopedAndUndevelopedReservesSalesOfMineralsInPlace / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_OilReservesMember | (1,286)us-gaap_ProvedDevelopedAndUndevelopedReservesSalesOfMineralsInPlace / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_OilReservesMember | (39)us-gaap_ProvedDevelopedAndUndevelopedReservesSalesOfMineralsInPlace / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_OilReservesMember
Revisions of previous estimates | (2,354)us-gaap_ProvedDevelopedAndUndevelopedReservesRevisionsOfPreviousEstimatesIncreaseDecrease / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_OilReservesMember | 354 us-gaap_ProvedDevelopedAndUndevelopedReservesRevisionsOfPreviousEstimatesIncreaseDecrease / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_OilReservesMember | 307 us-gaap_ProvedDevelopedAndUndevelopedReservesRevisionsOfPreviousEstimatesIncreaseDecrease / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_OilReservesMember
Balance at end of year | 43,918 us-gaap_ProvedDevelopedAndUndevelopedReservesNet / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_OilReservesMember | 41,019 us-gaap_ProvedDevelopedAndUndevelopedReservesNet / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_OilReservesMember | 33,453 us-gaap_ProvedDevelopedAndUndevelopedReservesNet / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_OilReservesMember
NGL (MBbls) |  |  |
Proved developed and undeveloped reserves: |  |  |
Balance at beginning of year | 6,602 us-gaap_ProvedDevelopedAndUndevelopedReservesNet / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasLiquidsReservesMember | 7,153 us-gaap_ProvedDevelopedAndUndevelopedReservesNet / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasLiquidsReservesMember | 7,342 us-gaap_ProvedDevelopedAndUndevelopedReservesNet / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasLiquidsReservesMember
Production | (609)us-gaap_ProvedDevelopedAndUndevelopedReservesProduction / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasLiquidsReservesMember | (781)us-gaap_ProvedDevelopedAndUndevelopedReservesProduction / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasLiquidsReservesMember | (828)us-gaap_ProvedDevelopedAndUndevelopedReservesProduction / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasLiquidsReservesMember
Extensions and discoveries | 3,634 us-gaap_ProvedDevelopedAndUndevelopedReservesExtensionsDiscoveriesAndAdditions / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasLiquidsReservesMember | 1,333 us-gaap_ProvedDevelopedAndUndevelopedReservesExtensionsDiscoveriesAndAdditions / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasLiquidsReservesMember | 1,817 us-gaap_ProvedDevelopedAndUndevelopedReservesExtensionsDiscoveriesAndAdditions / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasLiquidsReservesMember
Improved recovery | 0 us-gaap_ProvedDevelopedAndUndevelopedReservesImprovedRecovery / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasLiquidsReservesMember | 0 us-gaap_ProvedDevelopedAndUndevelopedReservesImprovedRecovery / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasLiquidsReservesMember | 0 us-gaap_ProvedDevelopedAndUndevelopedReservesImprovedRecovery / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasLiquidsReservesMember
Purchases of proved reserves | 0 us-gaap_ProvedDevelopedAndUndevelopedReservesPurchasesOfMineralsInPlace / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasLiquidsReservesMember | 0 us-gaap_ProvedDevelopedAndUndevelopedReservesPurchasesOfMineralsInPlace / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasLiquidsReservesMember | 0 us-gaap_ProvedDevelopedAndUndevelopedReservesPurchasesOfMineralsInPlace / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasLiquidsReservesMember
Sales of proved reserves | (3,109)us-gaap_ProvedDevelopedAndUndevelopedReservesSalesOfMineralsInPlace / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasLiquidsReservesMember | (25)us-gaap_ProvedDevelopedAndUndevelopedReservesSalesOfMineralsInPlace / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasLiquidsReservesMember | 0 us-gaap_ProvedDevelopedAndUndevelopedReservesSalesOfMineralsInPlace / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasLiquidsReservesMember
Revisions of previous estimates | 669 us-gaap_ProvedDevelopedAndUndevelopedReservesRevisionsOfPreviousEstimatesIncreaseDecrease / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasLiquidsReservesMember | (1,078)us-gaap_ProvedDevelopedAndUndevelopedReservesRevisionsOfPreviousEstimatesIncreaseDecrease / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasLiquidsReservesMember | (1,178)us-gaap_ProvedDevelopedAndUndevelopedReservesRevisionsOfPreviousEstimatesIncreaseDecrease / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasLiquidsReservesMember
Balance at end of year | 7,187 us-gaap_ProvedDevelopedAndUndevelopedReservesNet / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasLiquidsReservesMember | 6,602 us-gaap_ProvedDevelopedAndUndevelopedReservesNet / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasLiquidsReservesMember | 7,153 us-gaap_ProvedDevelopedAndUndevelopedReservesNet / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasLiquidsReservesMember
Natural Gas (MMcf) |  |  |
Proved developed and undeveloped reserves: |  |  |
Balance at beginning of year | 198,445 us-gaap_ProvedDevelopedAndUndevelopedReservesNet / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasReservesMember | 239,278 us-gaap_ProvedDevelopedAndUndevelopedReservesNet / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasReservesMember | 379,827 us-gaap_ProvedDevelopedAndUndevelopedReservesNet / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasReservesMember
Production | (20,822)us-gaap_ProvedDevelopedAndUndevelopedReservesProduction / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasReservesMember | (28,008)us-gaap_ProvedDevelopedAndUndevelopedReservesProduction / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasReservesMember | (33,214)us-gaap_ProvedDevelopedAndUndevelopedReservesProduction / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasReservesMember
Extensions and discoveries | 64,420 us-gaap_ProvedDevelopedAndUndevelopedReservesExtensionsDiscoveriesAndAdditions / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasReservesMember | 26,428 us-gaap_ProvedDevelopedAndUndevelopedReservesExtensionsDiscoveriesAndAdditions / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasReservesMember | 18,386 us-gaap_ProvedDevelopedAndUndevelopedReservesExtensionsDiscoveriesAndAdditions / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasReservesMember
Improved recovery | 0 us-gaap_ProvedDevelopedAndUndevelopedReservesImprovedRecovery / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasReservesMember | 0 us-gaap_ProvedDevelopedAndUndevelopedReservesImprovedRecovery / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasReservesMember | 0 us-gaap_ProvedDevelopedAndUndevelopedReservesImprovedRecovery / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasReservesMember
Purchases of proved reserves | 7,711 us-gaap_ProvedDevelopedAndUndevelopedReservesPurchasesOfMineralsInPlace / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasReservesMember | 0 us-gaap_ProvedDevelopedAndUndevelopedReservesPurchasesOfMineralsInPlace / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasReservesMember | 0 us-gaap_ProvedDevelopedAndUndevelopedReservesPurchasesOfMineralsInPlace / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasReservesMember
Sales of proved reserves | (40,451)us-gaap_ProvedDevelopedAndUndevelopedReservesSalesOfMineralsInPlace / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasReservesMember | (40,055)us-gaap_ProvedDevelopedAndUndevelopedReservesSalesOfMineralsInPlace / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasReservesMember | (2,307)us-gaap_ProvedDevelopedAndUndevelopedReservesSalesOfMineralsInPlace / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasReservesMember
Revisions of previous estimates | 35,708 us-gaap_ProvedDevelopedAndUndevelopedReservesRevisionsOfPreviousEstimatesIncreaseDecrease / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasReservesMember | 802 us-gaap_ProvedDevelopedAndUndevelopedReservesRevisionsOfPreviousEstimatesIncreaseDecrease / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasReservesMember | (123,414)us-gaap_ProvedDevelopedAndUndevelopedReservesRevisionsOfPreviousEstimatesIncreaseDecrease / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasReservesMember
Balance at end of year | 245,011 us-gaap_ProvedDevelopedAndUndevelopedReservesNet / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasReservesMember | 198,445 us-gaap_ProvedDevelopedAndUndevelopedReservesNet / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasReservesMember | 239,278 us-gaap_ProvedDevelopedAndUndevelopedReservesNet / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasReservesMember

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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=8451039&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=8451039&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=8451039&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=8451039&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=8451039&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=27015464&loc=d3e511914-122862 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 10 -Paragraph a -Subparagraph 2 -Article 4 + Details
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- Definition Production of proved reserves. + 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=8451039&loc=d3e61831-109447 + Details
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- Definition Revisions represent changes in previous estimates of proved reserves, either upward or downward, resulting from new information (except for an increase in proved acreage) normally obtained from development drilling and production history or resulting from change in economic factors. + 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 (a) -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e61831-109447 + Details
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- Definition Energy measure of decrease in quantity of proved reserve of oil and gas resulting from sale of mineral in place, 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 (f) -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e61831-109447 + Details
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