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Derivative Instruments Subject to master netting agreements (Details 4) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Derivative instruments [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
Commodity derivative instruments assets [Member] |  |
Derivative instruments [Line Items] |  |
Derivative asset, fair value, gross asset | 18,335 us-gaap_DerivativeFairValueOfDerivativeAsset / us-gaap_BalanceSheetLocationAxis = mdu_CommodityDerivativeInstrumentsAssetsMember | 1,950 us-gaap_DerivativeFairValueOfDerivativeAsset / us-gaap_BalanceSheetLocationAxis = mdu_CommodityDerivativeInstrumentsAssetsMember
Gross amount not offset | 0 mdu_DerivativeLiabilityFairValueGrossAmountNotOffset / us-gaap_BalanceSheetLocationAxis = mdu_CommodityDerivativeInstrumentsAssetsMember | (1,950)mdu_DerivativeLiabilityFairValueGrossAmountNotOffset / us-gaap_BalanceSheetLocationAxis = mdu_CommodityDerivativeInstrumentsAssetsMember
Derivative asset, fair value, net | 18,335 us-gaap_DerivativeFairValueOfDerivativeAssetAmountNotOffsetAgainstCollateral / us-gaap_BalanceSheetLocationAxis = mdu_CommodityDerivativeInstrumentsAssetsMember | 0 us-gaap_DerivativeFairValueOfDerivativeAssetAmountNotOffsetAgainstCollateral / us-gaap_BalanceSheetLocationAxis = mdu_CommodityDerivativeInstrumentsAssetsMember
Total assets [Member] |  |
Derivative instruments [Line Items] |  |
Derivative asset, fair value, gross asset | 18,335 us-gaap_DerivativeFairValueOfDerivativeAsset / us-gaap_BalanceSheetLocationAxis = mdu_TotalAssetsMember | 1,950 us-gaap_DerivativeFairValueOfDerivativeAsset / us-gaap_BalanceSheetLocationAxis = mdu_TotalAssetsMember
Gross amount not offset | 0 mdu_DerivativeLiabilityFairValueGrossAmountNotOffset / us-gaap_BalanceSheetLocationAxis = mdu_TotalAssetsMember | (1,950)mdu_DerivativeLiabilityFairValueGrossAmountNotOffset / us-gaap_BalanceSheetLocationAxis = mdu_TotalAssetsMember
Derivative asset, fair value, net | 18,335 us-gaap_DerivativeFairValueOfDerivativeAssetAmountNotOffsetAgainstCollateral / us-gaap_BalanceSheetLocationAxis = mdu_TotalAssetsMember | 0 us-gaap_DerivativeFairValueOfDerivativeAssetAmountNotOffsetAgainstCollateral / us-gaap_BalanceSheetLocationAxis = mdu_TotalAssetsMember
Commodity derivatives - liabilities [Member] |  |
Derivative instruments [Line Items] |  |
Derivative liability, fair value, gross liability |  | 7,483 us-gaap_DerivativeFairValueOfDerivativeLiability / us-gaap_BalanceSheetLocationAxis = mdu_CommodityDerivativesLiabilitiesMember
Gross amount not offset |  | (1,950)mdu_DerivativeAssetFairValueGrossAmountNotOffset / us-gaap_BalanceSheetLocationAxis = mdu_CommodityDerivativesLiabilitiesMember
Derivative liability, fair value, net |  | 5,533 us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral / us-gaap_BalanceSheetLocationAxis = mdu_CommodityDerivativesLiabilitiesMember
Total liabilities [Member] |  |
Derivative instruments [Line Items] |  |
Derivative liability, fair value, gross liability |  | 7,483 us-gaap_DerivativeFairValueOfDerivativeLiability / us-gaap_BalanceSheetLocationAxis = mdu_TotalLiabilitiesMember
Gross amount not offset |  | (1,950)mdu_DerivativeAssetFairValueGrossAmountNotOffset / us-gaap_BalanceSheetLocationAxis = mdu_TotalLiabilitiesMember
Derivative liability, fair value, net |  | $ 5,533 us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral / us-gaap_BalanceSheetLocationAxis = mdu_TotalLiabilitiesMember

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- Definition The gross amount as of the balance sheet date of the fair value of derivative assets that in accordance with the entity's accounting policy were not offset against the derivative liability under a master netting arrangement. + References No definition available. + Details
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- Definition The gross amount as of the balance sheet date of the fair value of derivative liabilities that in accordance with the entity's accounting policy were not offset against the derivative asset under a master netting arrangement. + References No definition available. + Details
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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, after offset of derivative liability, of 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, before offset against an obligation to return collateral under a master netting arrangement. 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 (c) -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 210 -SubTopic 20 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=36459792&loc=SL20226008-175313 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 9 -URI http://asc.fasb.org/extlink&oid=36459792&loc=SL20226006-175313 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=35706170&loc=d3e41678-113959 + 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, after offset of derivative asset, of 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, elected not to be and before offset against a right to receive collateral under a master netting arrangement. 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 (c) -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 210 -SubTopic 20 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=36459792&loc=SL20226008-175313 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 9 -URI http://asc.fasb.org/extlink&oid=36459792&loc=SL20226006-175313 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=35706170&loc=d3e41678-113959 + Details
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