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Commitments, contingencies and litigation (Details) (USD $) | 0 Months Ended | 3 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Aug. 06, 2012 | Jun. 30, 2012 | Sep. 30, 2010 | Dec. 31, 2014 | Dec. 31, 2013 | Aug. 31, 2011 | Oct. 31, 2010
Loss Contingencies [Line Items] |  |  |  |  |  |  |
Potential liabilities related to litigation and environmental matters |  |  |  | $ 27,600,000 us-gaap_LossContingencyAccrualAtCarryingValue | $ 29,500,000 us-gaap_LossContingencyAccrualAtCarryingValue |  |
Litigation related to construction materials [Member] |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |
Loss contingency, estimate of possible loss |  |  |  |  |  | 3,700,000 us-gaap_LossContingencyEstimateOfPossibleLoss / mdu_LossContingenciesByIndividualLitigationAxis = mdu_LitigationRelatedToConstructionMaterialsMember |
Additional amount per day of potential penalties |  |  |  |  |  | 5,000 mdu_AdditionalAmountPerDayOfPotentialPenalties / mdu_LossContingenciesByIndividualLitigationAxis = mdu_LitigationRelatedToConstructionMaterialsMember |
Former employee litigation [Member] |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |
Plaintiffs request compensatory damages | 23,800,000 us-gaap_LossContingencyDamagesSoughtValue / mdu_LossContingenciesByIndividualLitigationAxis = mdu_FormerEmployeeLitigationMemberMember |  |  |  |  |  |
Natural gas gathering operations [Member] |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |
Arbitration award |  |  |  |  |  |  | 26,600,000 mdu_ArbitrationAward / mdu_LossContingenciesByIndividualLitigationAxis = mdu_NaturalGasGatheringOperationsMember
Amount of expense (benefit) recorded in operation and maintenance expense related to natural gas gathering contract dispute, before tax |  | (24,100,000)mdu_AmountOfExpenseBenefitRecordedInOperationAndMaintenanceExpenseRelatedToNaturalGasGatheringContractDisputeBeforeTax / mdu_LossContingenciesByIndividualLitigationAxis = mdu_NaturalGasGatheringOperationsMember | 26,600,000 mdu_AmountOfExpenseBenefitRecordedInOperationAndMaintenanceExpenseRelatedToNaturalGasGatheringContractDisputeBeforeTax / mdu_LossContingenciesByIndividualLitigationAxis = mdu_NaturalGasGatheringOperationsMember |  |  |  |
Amount of expense (benefit) recorded in operation and maintenance expense related to natural gas gathering contract dispute, after tax |  | $ (15,000,000)mdu_AmountOfExpenseBenefitRecordedInOperationAndMaintenanceExpenseRelatedToNaturalGasGatheringContractDisputeAfterTax / mdu_LossContingenciesByIndividualLitigationAxis = mdu_NaturalGasGatheringOperationsMember | $ 16,500,000 mdu_AmountOfExpenseBenefitRecordedInOperationAndMaintenanceExpenseRelatedToNaturalGasGatheringContractDisputeAfterTax / mdu_LossContingenciesByIndividualLitigationAxis = mdu_NaturalGasGatheringOperationsMember |  |  |  |

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- Definition Amount of expense (benefit) recorded in operation and maintenance expense related to natural gas gathering contract dispute, after tax. + References No definition available. + Details
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- Definition Amount of expense (benefit) recorded in operation and maintenance expense related to natural gas gathering contract dispute, before tax. + References No definition available. + Details
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- Definition Amount of loss contingency liability. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14326-108349 + Details
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- Definition The value (monetary amount) of the award the plaintiff seeks in the legal matter. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14326-108349 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14435-108349 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14557-108349 + Details
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- Definition Reflects the estimated amount of loss from the specified contingency as of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14435-108349 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 9 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14557-108349 + Details
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