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Jointly owned facilities (Details) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Big Stone Station: |  |
Jointly owned facilities [Line Items] |  |
Utility plant in service | $ 64,283 us-gaap_JointlyOwnedUtilityPlantGrossOwnershipAmountOfPlantInService / us-gaap_PublicUtilityAxis = mdu_BigStoneStationMember | $ 63,890 us-gaap_JointlyOwnedUtilityPlantGrossOwnershipAmountOfPlantInService / us-gaap_PublicUtilityAxis = mdu_BigStoneStationMember
Less accumulated depreciation | 43,043 us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfPlantAccumulatedDepreciation / us-gaap_PublicUtilityAxis = mdu_BigStoneStationMember | 41,323 us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfPlantAccumulatedDepreciation / us-gaap_PublicUtilityAxis = mdu_BigStoneStationMember
Utility plant in services net | 21,240 us-gaap_JointlyOwnedUtilityPlantNetOwnershipAmount / us-gaap_PublicUtilityAxis = mdu_BigStoneStationMember | 22,567 us-gaap_JointlyOwnedUtilityPlantNetOwnershipAmount / us-gaap_PublicUtilityAxis = mdu_BigStoneStationMember
Coyote Station: |  |
Jointly owned facilities [Line Items] |  |
Utility plant in service | 138,810 us-gaap_JointlyOwnedUtilityPlantGrossOwnershipAmountOfPlantInService / us-gaap_PublicUtilityAxis = mdu_CoyoteStationMember | 138,261 us-gaap_JointlyOwnedUtilityPlantGrossOwnershipAmountOfPlantInService / us-gaap_PublicUtilityAxis = mdu_CoyoteStationMember
Less accumulated depreciation | 94,443 us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfPlantAccumulatedDepreciation / us-gaap_PublicUtilityAxis = mdu_CoyoteStationMember | 89,528 us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfPlantAccumulatedDepreciation / us-gaap_PublicUtilityAxis = mdu_CoyoteStationMember
Utility plant in services net | 44,367 us-gaap_JointlyOwnedUtilityPlantNetOwnershipAmount / us-gaap_PublicUtilityAxis = mdu_CoyoteStationMember | 48,733 us-gaap_JointlyOwnedUtilityPlantNetOwnershipAmount / us-gaap_PublicUtilityAxis = mdu_CoyoteStationMember
Wygen III: |  |
Jointly owned facilities [Line Items] |  |
Utility plant in service | 65,597 us-gaap_JointlyOwnedUtilityPlantGrossOwnershipAmountOfPlantInService / us-gaap_PublicUtilityAxis = mdu_WygenIiiMember | 64,332 us-gaap_JointlyOwnedUtilityPlantGrossOwnershipAmountOfPlantInService / us-gaap_PublicUtilityAxis = mdu_WygenIiiMember
Less accumulated depreciation | 5,928 us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfPlantAccumulatedDepreciation / us-gaap_PublicUtilityAxis = mdu_WygenIiiMember | 4,639 us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfPlantAccumulatedDepreciation / us-gaap_PublicUtilityAxis = mdu_WygenIiiMember
Utility plant in services net | $ 59,669 us-gaap_JointlyOwnedUtilityPlantNetOwnershipAmount / us-gaap_PublicUtilityAxis = mdu_WygenIiiMember | $ 59,693 us-gaap_JointlyOwnedUtilityPlantNetOwnershipAmount / us-gaap_PublicUtilityAxis = mdu_WygenIiiMember

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- Definition The period end amount owned by the reporting utility of a jointly owned utility plant in service. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 360 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 10.C) -URI http://asc.fasb.org/extlink&oid=6500719&loc=d3e659983-123033 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 10 -Section C + Details
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- Definition The total period end amount of plant in service and construction in progress net of accumulated depreciation for the utility's share of jointly owned electricity generation plant. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 360 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 10.C) -URI http://asc.fasb.org/extlink&oid=6500719&loc=d3e659983-123033 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 10 -Section C + Details
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- Definition The period end amount of accumulated depreciation in a jointly owned electricity generation plant. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 360 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 10.C) -URI http://asc.fasb.org/extlink&oid=6500719&loc=d3e659983-123033 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 10 -Section C + Details
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