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Long-term debt outstanding (Details 4) (USD $) | Dec. 31, 2014 | Dec. 31, 2013
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Long-term debt maturities [Abstract] |  |
2015 | $ 269,400,000 us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths |
2016 | 293,800,000 us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo |
2017 | 51,000,000 us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree |
2018 | 148,200,000 us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFour |
2019 | 345,700,000 us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFive |
Thereafter | 986,600,000 us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalAfterYearFive |
Senior notes [Member] |  |
Long term debt outstanding [Abstract] |  |
Weighted average interest rate | 5.43%us-gaap_LongtermDebtWeightedAverageInterestRate / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember |
Commercial paper [Member] |  |
Long term debt outstanding [Abstract] |  |
Weighted average interest rate | 0.37%us-gaap_LongtermDebtWeightedAverageInterestRate / us-gaap_LongtermDebtTypeAxis = us-gaap_CommercialPaperMember |
Term loan agreements [Member] |  |
Long term debt outstanding [Abstract] |  |
Weighted average interest rate | 2.12%us-gaap_LongtermDebtWeightedAverageInterestRate / us-gaap_LongtermDebtTypeAxis = mdu_TermloanagreementsMember |
Medium-term notes [Member] |  |
Long term debt outstanding [Abstract] |  |
Weighted average interest rate | 7.32%us-gaap_LongtermDebtWeightedAverageInterestRate / us-gaap_LongtermDebtTypeAxis = us-gaap_MediumTermNotesMember |
Other notes [Member] |  |
Long term debt outstanding [Abstract] |  |
Weighted average interest rate | 5.23%us-gaap_LongtermDebtWeightedAverageInterestRate / us-gaap_LongtermDebtTypeAxis = us-gaap_NotesPayableToBanksMember |
Loans payable [Member] |  |
Long term debt outstanding [Abstract] |  |
Weighted average interest rate | 3.47%us-gaap_LongtermDebtWeightedAverageInterestRate / us-gaap_LongtermDebtTypeAxis = us-gaap_LoansPayableMember |
Long-term debt [Member] |  |
Long term debt outstanding [Abstract] |  |
Long-term debt | 2,094,727,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember | 1,854,563,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember
Less current maturities | 269,449,000 us-gaap_LongTermDebtCurrent / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember | 12,277,000 us-gaap_LongTermDebtCurrent / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember
Long-term debt | 1,825,278,000 us-gaap_LongTermDebtNoncurrent / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember | 1,842,286,000 us-gaap_LongTermDebtNoncurrent / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember
Long-term debt [Member] | Senior notes [Member] |  |
Long term debt outstanding [Abstract] |  |
Long-term debt | 1,636,662,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember | 1,545,078,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember
Long-term debt [Member] | Commercial paper [Member] |  |
Long term debt outstanding [Abstract] |  |
Long-term debt | 288,500,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / us-gaap_LongtermDebtTypeAxis = us-gaap_CommercialPaperMember | 153,924,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / us-gaap_LongtermDebtTypeAxis = us-gaap_CommercialPaperMember
Long-term debt [Member] | Term loan agreements [Member] |  |
Long term debt outstanding [Abstract] |  |
Long-term debt | 72,000,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / us-gaap_LongtermDebtTypeAxis = mdu_TermloanagreementsMember | 75,000,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / us-gaap_LongtermDebtTypeAxis = mdu_TermloanagreementsMember
Long-term debt [Member] | Medium-term notes [Member] |  |
Long term debt outstanding [Abstract] |  |
Long-term debt | 35,000,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / us-gaap_LongtermDebtTypeAxis = us-gaap_MediumTermNotesMember | 35,000,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / us-gaap_LongtermDebtTypeAxis = us-gaap_MediumTermNotesMember
Long-term debt [Member] | Other notes [Member] |  |
Long term debt outstanding [Abstract] |  |
Long-term debt | 39,662,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / us-gaap_LongtermDebtTypeAxis = us-gaap_NotesPayableToBanksMember | 39,863,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / us-gaap_LongtermDebtTypeAxis = us-gaap_NotesPayableToBanksMember
Long-term debt [Member] | Loans payable [Member] |  |
Long term debt outstanding [Abstract] |  |
Long-term debt | 22,939,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / us-gaap_LongtermDebtTypeAxis = us-gaap_LoansPayableMember | 5,701,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / us-gaap_LongtermDebtTypeAxis = us-gaap_LoansPayableMember
Long-term debt [Member] | Discount on commercial paper program [Member] |  |
Long term debt outstanding [Abstract] |  |
Long-term debt | $ (36,000)us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / us-gaap_LongtermDebtTypeAxis = mdu_DiscountOnCommercialPaperProgramMember | $ (3,000)us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / us-gaap_LongtermDebtTypeAxis = mdu_DiscountOnCommercialPaperProgramMember

X
- Definition Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 7 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 9 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 20, 22 -Article 5 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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- Definition Amount of long-term debt, after unamortized discount or premium, scheduled to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations. + 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 210.5-02.19,20) -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 20 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19 -Article 5 + Details
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- Definition Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-04.(a)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24072-122690 + Details
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- Definition Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-04.(a)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24072-122690 + Details
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- Definition Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-04.(a)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24072-122690 + Details
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- Definition Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-04.(a)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24072-122690 + Details
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- Definition Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-04.(a)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24072-122690 + Details
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- Definition Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-04.(a)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24072-122690 + Details
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