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Proved reserves (Details 5) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 MBoe | Dec. 31, 2013 MBoe | Dec. 31, 2012 MBoe | Dec. 31, 2011 MBoe
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Proved Developed and Undeveloped Reserves [Abstract] |  |  |  |
Proved developed reserves, energy | 65,086 us-gaap_ProvedDevelopedReservesBOE1 | 66,140 us-gaap_ProvedDevelopedReservesBOE1 | 69,131 us-gaap_ProvedDevelopedReservesBOE1 |
Proved undeveloped reserves, energy | 26,854 us-gaap_ProvedUndevelopedReserveBOE1 | 14,555 us-gaap_ProvedUndevelopedReserveBOE1 | 11,355 us-gaap_ProvedUndevelopedReserveBOE1 |
Balance at end of year, energy | 91,940 us-gaap_ProvedDevelopedAndUndevelopedReserveNetEnergy | 80,695 us-gaap_ProvedDevelopedAndUndevelopedReserveNetEnergy | 80,486 us-gaap_ProvedDevelopedAndUndevelopedReserveNetEnergy | 97,651 us-gaap_ProvedDevelopedAndUndevelopedReserveNetEnergy
Changes in proved and undeveloped reserves [Abstract] |  |  |  |
Change in PUD reserve | 12,300 mdu_ChangeInPudReserve |  |  |
New PUD reserves | 21,200 mdu_NewPudReserves |  |  |
Acquired PUD Reserves | 3,900 mdu_AmountofAcquiredPUDReserves |  |  |
Converting PUD reserves | 3,700 mdu_ConvertingPUDreserves |  |  |
Divested PUD Reserves | 6,400 mdu_DivestedPUDReserves |  |  |
Revision to PUD reserves | (2,700)mdu_RevisiontoPUDreserves |  |  |
Drilling, completion and future development costs [Abstract] |  |  |  |
Drilling and completion capital | $ 98.3 mdu_DrillingAndCompletionCapital |  |  |
Future development costs estimated to be spent to develop PUD reserves year one | 141.8 mdu_FutureDevelopmentCostsEstimatedToBeSpentToDevelopPudReservesYearOne |  |  |
Future development costs estimated to be spent to develop PUD reserves year two | 157.4 mdu_FutureDevelopmentCostsEstimatedToBeSpentToDevelopPudReservesYearTwo |  |  |
Future development costs estimated to be spent to develop PUD reserves year three | $ 120.9 mdu_FutureDevelopmentCostsEstimatedToBeSpentToDevelopPudReservesYearThree |  |  |
Oil (MBbls) |  |  |  |
Proved Developed and Undeveloped Reserves [Abstract] |  |  |  |
Proved developed reserves, volume | 30,130 us-gaap_ProvedDevelopedReservesVolume / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_OilReservesMember | 31,394 us-gaap_ProvedDevelopedReservesVolume / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_OilReservesMember | 27,412 us-gaap_ProvedDevelopedReservesVolume / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_OilReservesMember |
Proved undeveloped reserves, volume | 13,788 us-gaap_ProvedUndevelopedReserveVolume / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_OilReservesMember | 9,625 us-gaap_ProvedUndevelopedReserveVolume / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_OilReservesMember | 6,041 us-gaap_ProvedUndevelopedReserveVolume / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_OilReservesMember |
Balance at end of year, volume | 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 | 27,005 us-gaap_ProvedDevelopedAndUndevelopedReservesNet / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_OilReservesMember
NGL (MBbls) |  |  |  |
Proved Developed and Undeveloped Reserves [Abstract] |  |  |  |
Proved developed reserves, volume | 4,217 us-gaap_ProvedDevelopedReservesVolume / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasLiquidsReservesMember | 5,322 us-gaap_ProvedDevelopedReservesVolume / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasLiquidsReservesMember | 5,342 us-gaap_ProvedDevelopedReservesVolume / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasLiquidsReservesMember |
Proved undeveloped reserves, volume | 2,970 us-gaap_ProvedUndevelopedReserveVolume / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasLiquidsReservesMember | 1,280 us-gaap_ProvedUndevelopedReserveVolume / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasLiquidsReservesMember | 1,811 us-gaap_ProvedUndevelopedReserveVolume / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasLiquidsReservesMember |
Balance at end of year, volume | 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 | 7,342 us-gaap_ProvedDevelopedAndUndevelopedReservesNet / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasLiquidsReservesMember
Natural Gas (MMcf) |  |  |  |
Proved Developed and Undeveloped Reserves [Abstract] |  |  |  |
Proved developed reserves, volume | 184,437 us-gaap_ProvedDevelopedReservesVolume / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasReservesMember | 176,546 us-gaap_ProvedDevelopedReservesVolume / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasReservesMember | 218,259 us-gaap_ProvedDevelopedReservesVolume / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasReservesMember |
Proved undeveloped reserves, volume | 60,574 us-gaap_ProvedUndevelopedReserveVolume / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasReservesMember | 21,899 us-gaap_ProvedUndevelopedReserveVolume / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasReservesMember | 21,019 us-gaap_ProvedUndevelopedReserveVolume / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasReservesMember |
Balance at end of year, volume | 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 | 379,827 us-gaap_ProvedDevelopedAndUndevelopedReservesNet / us-gaap_ReserveQuantitiesByTypeOfReserveAxis = us-gaap_NaturalGasReservesMember

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- Definition Converting PUD reserves. + References No definition available. + Details
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- Definition Amount of divested PUD reserves. + References No definition available. + Details
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- Definition Drilling and completion capital required to convert PUD reserves into proved developed reserves. + References No definition available. + Details
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- Definition Future development costs estimated to be spent to develop PUD reserves year one. + References No definition available. + Details
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- Definition Future development costs estimated to be spent to develop PUD reserves year three. + References No definition available. + Details
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- Definition Future development costs estimated to be spent to develop PUD reserves year two. + References No definition available. + Details
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- Definition Amount of new PUD reserves. + References No definition available. + Details
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- Definition Amount of revision to PUD reserves. + References No definition available. + Details
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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=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 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 4 -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e61797-109447 Reference 2: 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=8451039&loc=d3e61884-109447 + Details
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- Definition Energy measure of proved undeveloped 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 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 4 -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e61797-109447 Reference 2: 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=8451039&loc=d3e61884-109447 + Details
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