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Variable interest entities (Details 6) (USD $) | Dec. 31, 2014 | Dec. 01, 2014 | Dec. 31, 2013
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Dakota Prairie Refining, LLC [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Percent of ownership | 50.00%mdu_PercentOfOwnership / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember |  |
Total project costs | $ 300,000,000 mdu_Totalprojectcosts / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember |  |
Portion of capital commitment | 150,000,000 mdu_PortionOfCapitalCommitment / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember |  |
Partner portion of capital commitment | 75,000,000 mdu_PartnerPortionOfCapitalCommitment / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember |  |
Excess capital commitments | 300,000,000 mdu_ExcessCapitalCommitments / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember |  |
Estimated project cost | 400,000,000 mdu_ProjectCostEstimate / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember |  |
Term loan for project debt financing | 75,000,000 mdu_TermLoanForProjectDebtFinancing / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember |  |
Facility limit, maximum borrowing capacity |  | 50,000,000 us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember |
Fuel contract [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Variable interest entity, reporting entity involvement, maximum loss exposure, amount | 15,800,000 us-gaap_VariableInterestEntityEntityMaximumLossExposureAmount / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_FuelContractMember |  |
WBI Holdings [Member] | Dakota Prairie Refining, LLC [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Percent of credit agreement guaranteed |  | 50.00%mdu_PercentofCreditAgreementGuaranteed / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = mdu_WBIHoldingsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember |
Noncontrolling interest | Dakota Prairie Refining, LLC [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Percent of credit agreement guaranteed |  | 50.00%mdu_PercentofCreditAgreementGuaranteed / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = us-gaap_NoncontrollingInterestMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember |
Cash and cash equivalents [Member] | Dakota Prairie Refining, LLC [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Variable interest entity, consolidated, carrying amount, assets | 21,376,000 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets / us-gaap_BalanceSheetLocationAxis = us-gaap_CashAndCashEquivalentsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember |  | 4,774,000 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets / us-gaap_BalanceSheetLocationAxis = us-gaap_CashAndCashEquivalentsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember
Accounts receivable [Member] | Dakota Prairie Refining, LLC [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Variable interest entity, consolidated, carrying amount, assets | 2,759,000 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets / us-gaap_BalanceSheetLocationAxis = us-gaap_AccountsReceivableMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember |  | 0 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets / us-gaap_BalanceSheetLocationAxis = us-gaap_AccountsReceivableMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember
Inventories [Member] | Dakota Prairie Refining, LLC [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Variable interest entity, consolidated, carrying amount, assets | 5,311,000 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets / us-gaap_BalanceSheetLocationAxis = us-gaap_InventoriesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember |  | 0 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets / us-gaap_BalanceSheetLocationAxis = us-gaap_InventoriesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember
Other current assets [Member] | Dakota Prairie Refining, LLC [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Variable interest entity, consolidated, carrying amount, assets | 4,019,000 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherCurrentAssetsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember |  | 26,000 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherCurrentAssetsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember
Total current assets [Member] | Dakota Prairie Refining, LLC [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Variable interest entity, consolidated, carrying amount, assets | 33,465,000 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets / us-gaap_BalanceSheetLocationAxis = mdu_TotalCurrentAssetsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember |  | 4,800,000 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets / us-gaap_BalanceSheetLocationAxis = mdu_TotalCurrentAssetsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember
Net property, plant and equipment [Member] | Dakota Prairie Refining, LLC [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Variable interest entity, consolidated, carrying amount, assets | 398,984,000 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets / us-gaap_BalanceSheetLocationAxis = mdu_NetPropertyPlantAndEquipmentMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember |  | 172,073,000 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets / us-gaap_BalanceSheetLocationAxis = mdu_NetPropertyPlantAndEquipmentMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember
Other [Member] | Dakota Prairie Refining, LLC [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Variable interest entity, consolidated, carrying amount, assets | 3,400,000 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherNoncurrentAssetsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember |  | 0 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherNoncurrentAssetsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember
Total deferred charges and other assets [Member] | Dakota Prairie Refining, LLC [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Variable interest entity, consolidated, carrying amount, assets | 3,400,000 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets / us-gaap_BalanceSheetLocationAxis = mdu_TotalDeferredChargesandOtherAssetsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember |  | 0 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets / us-gaap_BalanceSheetLocationAxis = mdu_TotalDeferredChargesandOtherAssetsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember
Total assets [Member] | Dakota Prairie Refining, LLC [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Variable interest entity, consolidated, carrying amount, assets | 435,849,000 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets / us-gaap_BalanceSheetLocationAxis = us-gaap_AssetsTotalMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember |  | 176,873,000 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets / us-gaap_BalanceSheetLocationAxis = us-gaap_AssetsTotalMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember
Long-term debt due within one year [Member] | Dakota Prairie Refining, LLC [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Variable interest entity, consolidated, carrying amount, liabilities | 3,000,000 us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities / us-gaap_BalanceSheetLocationAxis = mdu_LongtermdebtduewithinoneyearMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember |  | 3,000,000 us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities / us-gaap_BalanceSheetLocationAxis = mdu_LongtermdebtduewithinoneyearMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember
Accounts payable [Member] | Dakota Prairie Refining, LLC [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Variable interest entity, consolidated, carrying amount, liabilities | 55,089,000 us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities / us-gaap_BalanceSheetLocationAxis = us-gaap_AccountsPayableMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember |  | 8,904,000 us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities / us-gaap_BalanceSheetLocationAxis = us-gaap_AccountsPayableMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember
Taxes payable [Member] | Dakota Prairie Refining, LLC [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Variable interest entity, consolidated, carrying amount, liabilities | 648,000 us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities / us-gaap_BalanceSheetLocationAxis = mdu_TaxesPayableMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember |  | 5,000 us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities / us-gaap_BalanceSheetLocationAxis = mdu_TaxesPayableMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember
Accrued compensation [Member] | Dakota Prairie Refining, LLC [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Variable interest entity, consolidated, carrying amount, liabilities | 727,000 us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities / us-gaap_BalanceSheetLocationAxis = mdu_AccruedCompensationMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember |  | 26,000 us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities / us-gaap_BalanceSheetLocationAxis = mdu_AccruedCompensationMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember
Other accrued liabilities [Member] | Dakota Prairie Refining, LLC [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Variable interest entity, consolidated, carrying amount, liabilities | 899,000 us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities / us-gaap_BalanceSheetLocationAxis = mdu_OtherAccruedLiabilitiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember |  | 461,000 us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities / us-gaap_BalanceSheetLocationAxis = mdu_OtherAccruedLiabilitiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember
Total current liabilities [Member] | Dakota Prairie Refining, LLC [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Variable interest entity, consolidated, carrying amount, liabilities | 60,363,000 us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities / us-gaap_BalanceSheetLocationAxis = mdu_TotalcurrentliabilitiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember |  | 12,396,000 us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities / us-gaap_BalanceSheetLocationAxis = mdu_TotalcurrentliabilitiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember
Long-term debt [Member] | Dakota Prairie Refining, LLC [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Variable interest entity, consolidated, carrying amount, liabilities | 69,000,000 us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities / us-gaap_BalanceSheetLocationAxis = us-gaap_LongTermDebtMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember |  | 72,000,000 us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities / us-gaap_BalanceSheetLocationAxis = us-gaap_LongTermDebtMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember
Total liabilities [Member] | Dakota Prairie Refining, LLC [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Variable interest entity, consolidated, carrying amount, liabilities | $ 129,363,000 us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities / us-gaap_BalanceSheetLocationAxis = mdu_TotalLiabilitiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember |  | $ 84,396,000 us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities / us-gaap_BalanceSheetLocationAxis = mdu_TotalLiabilitiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mdu_DakotaPrairieRefiningLlcMember

X
- Definition Excess capital commitments above this amount will be shared equally. + References No definition available. + Details
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- Definition Partner portion of capital commitment. + References No definition available. + Details
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- Definition Percent of credit agreement guaranteed. + References No definition available. + Details
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- Definition Percent of ownership. + References No definition available. + Details
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- Definition Portion of capital commitment. + References No definition available. + Details
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- Definition Project cost estimate + References No definition available. + Details
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- Definition Amount of term loan entered into to finance the project. + References No definition available. + Details
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- Definition Total project costs. + References No definition available. + Details
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- Definition Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 22 -Article 5 + Details
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- Definition The carrying amount of the consolidated Variable Interest Entity's assets included in the reporting entity's statement of financial position. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (bb) -URI http://asc.fasb.org/extlink&oid=28200181&loc=d3e5710-111685 + Details
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- Definition The carrying amount of the consolidated Variable Interest Entity's liabilities included in the reporting entity's statement of financial position. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (bb) -URI http://asc.fasb.org/extlink&oid=28200181&loc=d3e5710-111685 + Details
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- Definition The reporting entity's maximum amount of exposure to loss as a result of its involvement with the Variable Interest Entity (VIE). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph b -URI http://asc.fasb.org/extlink&oid=28200181&loc=d3e5728-111685 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=28200181&loc=d3e5747-111685 + Details
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