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Common Stock (Details) (USD $) Share data in Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Dividends declared per common share	$ 0.7150us-gaap_CommonStockDividendsPerShareDeclared	$ 0.6950us-gaap_CommonStockDividendsPerShareDeclared	$ 0.6750us-gaap_CommonStockDividendsPerShareDeclared
Company's subsidiaries net assets restricted from being used to transfer funds to the Company	$ 2,300,000,000us-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	$ 259,000,000mdu_CompanysExcludingItsSubsidiariesNetAssetsRestrictedFromUseForDividendPayments
MDU Resources Group, Inc. [Member]
Shares of common stock reserved for original issuance under the Stock Purchase Plan and K-Plan (in shares)	14.9us-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

Company's (excluding its subsidiaries) net assets restricted from use for dividend payments.

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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

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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 210.5-02.29)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Aggregate dividends declared during the period for each share of common stock outstanding.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Article 3



Reference 2: http://www.xbrl.org/2003/role/presentationRef

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-Name Accounting Standards Codification

-Topic 505

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-Paragraph 1

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