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Common Stock (Details) (USD $) Share data in Millions, except Per Share data, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Dividends declared per common share | $ 0.7150 us-gaap_CommonStockDividendsPerShareDeclared | $ 0.6950 us-gaap_CommonStockDividendsPerShareDeclared | $ 0.6750 us-gaap_CommonStockDividendsPerShareDeclared
Company's subsidiaries net assets restricted from being used to transfer funds to the Company | $ 2,300,000,000 us-gaap_AmountOfRestrictedNetAssetsForConsolidatedAndUnconsolidatedSubsidiaries |  |
Credit agreement limitation on company ratio of funded debt to capitalization (excluding subsidiaries) | 65.00%mdu_RatioOfEarningsBeforeInterestAndTaxesToInterestExpenseAsSpecifiedInDebtConvenants |  |
Company's (excluding its subsidiaries) net assets restricted from use for dividend payments | $ 259,000,000 mdu_CompanysExcludingItsSubsidiariesNetAssetsRestrictedFromUseForDividendPayments |  |
MDU Resources Group, Inc. [Member] |  |  |
Shares of common stock reserved for original issuance under the Stock Purchase Plan and K-Plan (in shares) | 14.9 us-gaap_CommonStockCapitalSharesReservedForFutureIssuance / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  |
Centennial [Member] |  |  |
Maximum distributions to the company as a percentage of consolidated net income after taxes, excluding noncash write-downs, for the preceding fiscal year (in hundredths) | 100.00%mdu_MaximumDistributionsToCompanyAsPercentageOfConsolidatedNetIncomeAfterTaxesExcludingNoncashWriteDownsForPrecedingFiscalYearMaximum / dei_LegalEntityAxis = mdu_CentennialMember |  |

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- Definition Amount of restricted net assets of consolidated and unconsolidated subsidiaries as of the end of the most recently completed fiscal year. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(e)(3)(ii)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph e -Subparagraph 3 -Article 4 + Details
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