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Derivative Instruments Derivative instruments (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Mar. 31, 2013
Derivative instruments [Line Items]
Amount of unrealized gain (loss) on hedge at de-designation date to be reclassified into AOCI			$ 1,800,000mdu_AmountofunrealizedgainlossonhedgeatdedesignationdatetobereclassifiedintoAOCI
Derivatives with credit-risk-related contingent feature in net liability position, aggregate fair value		7,500,000us-gaap_DerivativeNetLiabilityPositionAggregateFairValue
Assets needed for immediate settlement aggregate fair value for cash flow heding instruments		7,500,000us-gaap_AssetsNeededForImmediateSettlementAggregateFairValue
Cash flow hedge gain (loss) to be reclassified within twelve months	$ (400,000)us-gaap_CashFlowHedgeGainLossToBeReclassifiedWithinTwelveMonths
Oil swap and or collar [Member]
Derivative instruments [Line Items]
Derivative, nonmonetary notional amount	270,000invest_DerivativeNonmonetaryNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mdu_OilSwapAndOrCollarMember	2,900,000invest_DerivativeNonmonetaryNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mdu_OilSwapAndOrCollarMember
Natural gas swap or and collar [Member]
Derivative instruments [Line Items]
Derivative, nonmonetary notional amount	5,000,000invest_DerivativeNonmonetaryNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mdu_NaturalGasSwapAndorCollarMember	18,300,000invest_DerivativeNonmonetaryNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mdu_NaturalGasSwapAndorCollarMember

X

- Definition

Aggregate notional amount of derivative expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 12

-Section 13

-Sentence Column B

+ Details

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X

- Definition

Amount of unrealized gain (loss) on hedge at de-designation date to be reclassified into accumulated other comprehensive income over time.

+ References

No definition available.

+ Details

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Namespace Prefix:	mdu_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

The aggregate fair value of assets needed to immediately settle the derivative instruments with credit-risk-related contingent features if the credit-risk-related contingent features were triggered at the end of the reporting period.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4H

-Subparagraph (f)

-URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624258-113959

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

The estimated net amount of existing gains or losses on cash flow hedges at the reporting date expected to be reclassified to earnings within the next 12 months.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 30

-Section 50

-Paragraph 1

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=35736893&loc=d3e80748-113994

+ Details

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- Details

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X

- Definition

The aggregate fair value amounts of derivative instruments that contain credit-risk-related contingent features that are in a net liability position at the end of the reporting period. For nonderivative instruments that are designated and qualify as hedging instruments, the fair value amounts are the carrying value of the nonderivative hedging instrument, including the adjustment for the foreign currency transaction gain (loss) on that instrument.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4H

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624258-113959

+ Details

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Namespace Prefix:	us-gaap_
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- Details

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- Details

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