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Short-term borrowings (Details 2) (Revolving credit facility [Member], Short-term debt [Member], Dakota Prairie Refining, LLC [Member], USD $) In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 01, 2014
Revolving credit facility [Member] | Short-term debt [Member] | Dakota Prairie Refining, LLC [Member]
Short-term debt [Line Items]
Facility limit, maximum borrowing capacity	$ 50.0us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_DebtInstrumentAxis = us-gaap_ShortTermDebtMember / dei_LegalEntityAxis = mdu_DakotaPrairieRefiningLlcMember	[1]	$ 50.0us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_DebtInstrumentAxis = us-gaap_ShortTermDebtMember / dei_LegalEntityAxis = mdu_DakotaPrairieRefiningLlcMember
Ratio of total debt to total capitalization as specified in debt convenants.	65.00%mdu_RatioOfTotalDebtToTotalCapitalizationAsSpecifiedInDebtConvenants / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_DebtInstrumentAxis = us-gaap_ShortTermDebtMember / dei_LegalEntityAxis = mdu_DakotaPrairieRefiningLlcMember
Ratio of total funded debt to total capitalization as specified in debt covenants	65.00%mdu_Ratiooftotalfundeddebttototalcapitalizationasspecifiedindebtcovenants / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_DebtInstrumentAxis = us-gaap_ShortTermDebtMember / dei_LegalEntityAxis = mdu_DakotaPrairieRefiningLlcMember
[1] Certain provisions allow for increased borrowings up to a maximum of $75.0 million.

X

- Definition

Ratio of total debt to total capitalization as specified in debt convenants.

+ References

No definition available.

+ Details

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X

- Definition

Ratio of total funded debt to total capitalization as specified in debt covenants.

+ References

No definition available.

+ Details

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Namespace Prefix:	mdu_
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X

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ 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(b),22(b))

-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 19, 22

-Article 5

+ Details

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

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