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Available-for-sale securities (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Available-for-sale securities [Line Items]
Investments used to satisfy obligations under nonqualified benefit plans	$ 65,800,000mdu_InvestmentsUsedToSatisfyObligationsUnderNonqualifiedBenefitPlans	$ 62,400,000mdu_InvestmentsUsedToSatisfyObligationsUnderNonqualifiedBenefitPlans
Net unrealized gain (loss) of investments used to satify obligations under nonqualified benefit plans	3,400,000mdu_NetUnrealizedGainLossOfInvestmentsUsedToSatifyObligationsUnderNonqualifiedBenefitPlans	13,500,000mdu_NetUnrealizedGainLossOfInvestmentsUsedToSatifyObligationsUnderNonqualifiedBenefitPlans	5,200,000mdu_NetUnrealizedGainLossOfInvestmentsUsedToSatifyObligationsUnderNonqualifiedBenefitPlans
Available-for-sale securities:
Available-for-sale securities [Abstract]
Available-for-sale securities, amortized cost basis	10,168,000us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AvailableforsaleSecuritiesMember	10,057,000us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AvailableforsaleSecuritiesMember
Available-for-sale securities, gross unrealized gains	60,000us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AvailableforsaleSecuritiesMember	84,000us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AvailableforsaleSecuritiesMember
Available-for-sale securities, gross unrealized losses	(37,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AvailableforsaleSecuritiesMember	(31,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AvailableforsaleSecuritiesMember
Available-for-sale securities	10,191,000us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AvailableforsaleSecuritiesMember	10,110,000us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AvailableforsaleSecuritiesMember
Mortgage-backed securities
Available-for-sale securities [Abstract]
Available-for-sale securities, amortized cost basis	6,594,000us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember	8,151,000us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Available-for-sale securities, gross unrealized gains	60,000us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember	69,000us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Available-for-sale securities, gross unrealized losses	(18,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember	(27,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Available-for-sale securities	6,636,000us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember	8,193,000us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
U.S. Treasury securities
Available-for-sale securities [Abstract]
Available-for-sale securities, amortized cost basis	3,574,000us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasurySecuritiesMember	1,906,000us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasurySecuritiesMember
Available-for-sale securities, gross unrealized gains	0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasurySecuritiesMember	15,000us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasurySecuritiesMember
Available-for-sale securities, gross unrealized losses	(19,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasurySecuritiesMember	(4,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasurySecuritiesMember
Available-for-sale securities	$ 3,555,000us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasurySecuritiesMember	$ 1,917,000us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasurySecuritiesMember

X

- Definition

Investments used to satisfy obligations under unfunded, nonqualified benefit plans for executive officers and certain key management employees.

+ References

No definition available.

+ Details

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X

- Definition

Net unrealized gain (loss) of investments used to satify obligations under unfunded, nonqualified benefit plans for executive officers and certain key management employees.

+ References

No definition available.

+ Details

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X

- Definition

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 5

-Subparagraph (aa)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563



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-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562

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- Definition

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

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- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

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- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ 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 210.5-02.2)

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

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