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Fair value derivative instruments (Details 3) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Derivatives, fair value [Line Items] |  |
Derivative asset, fair value, gross asset | $ 18,335 us-gaap_DerivativeFairValueOfDerivativeAsset | $ 1,950 us-gaap_DerivativeFairValueOfDerivativeAsset
Derivative liability, fair value, gross liability | 0 us-gaap_DerivativeFairValueOfDerivativeLiability | 7,483 us-gaap_DerivativeFairValueOfDerivativeLiability
Not designated as hedging instruments [Member] | Commodity derivatives | Commodity derivative instruments assets, current [Member] |  |
Derivatives, fair value [Line Items] |  |
Other derivatives not designated as hedging instruments assets at fair value | 18,335 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsAssetAtFairValue / us-gaap_BalanceSheetLocationAxis = mdu_CommodityDerivativeInstrumentsAssetsCurrentMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | 1,447 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsAssetAtFairValue / us-gaap_BalanceSheetLocationAxis = mdu_CommodityDerivativeInstrumentsAssetsCurrentMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember
Not designated as hedging instruments [Member] | Commodity derivatives | Other assets noncurrent [Member] |  |
Derivatives, fair value [Line Items] |  |
Other derivatives not designated as hedging instruments assets at fair value | 0 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsAssetAtFairValue / us-gaap_BalanceSheetLocationAxis = mdu_OtherAssetsNoncurrentMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | 503 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsAssetAtFairValue / us-gaap_BalanceSheetLocationAxis = mdu_OtherAssetsNoncurrentMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember
Not designated as hedging instruments [Member] | Commodity derivatives | Commodity derivative instruments liabilities [Member] |  |
Derivatives, fair value [Line Items] |  |
Other derivatives not designated as hedging instruments liabilities at fair value | $ 0 us-gaap_OtherDerivativesNotDesignatedAsHedgingInstrumentsLiabilitiesAtFairValue / us-gaap_BalanceSheetLocationAxis = mdu_CommodityDerivativeInstrumentsLiabilitiesCurrentMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | $ 7,483 us-gaap_OtherDerivativesNotDesignatedAsHedgingInstrumentsLiabilitiesAtFairValue / us-gaap_BalanceSheetLocationAxis = mdu_CommodityDerivativeInstrumentsLiabilitiesCurrentMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember

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- Definition Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=34720828&loc=SL20225862-175312 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4B -Subparagraph (a),(c) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624163-113959 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=36459792&loc=SL20226000-175313 + Details
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- Definition Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=34720828&loc=SL20225862-175312 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4B -Subparagraph (a),(c) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624163-113959 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=36459792&loc=SL20226000-175313 + Details
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- Definition Fair value as of the balance sheet date of derivative instrument not designated as hedging instrument and classified as an asset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4B -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624163-113959 + Details
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- Definition Fair value of all derivative liabilities not designated as a hedging instrument. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13433-108611 + Details
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- Details
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