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Multiemployer other postretirement and defined contribution plans (Details 15) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Multiemployer Plans, Defined Contribution [Member]
Multiemployer Plans [Line Items]
Multiemployer plan, period contributions	$ 22,000us-gaap_MultiemployerPlanPeriodContributions / us-gaap_DefinedContributionPlanNameAxis = mdu_MultiemployerPlansDefinedContributionMember	$ 20,600us-gaap_MultiemployerPlanPeriodContributions / us-gaap_DefinedContributionPlanNameAxis = mdu_MultiemployerPlansDefinedContributionMember	$ 18,700us-gaap_MultiemployerPlanPeriodContributions / us-gaap_DefinedContributionPlanNameAxis = mdu_MultiemployerPlansDefinedContributionMember
Multiemployer Plans, Postretirement Benefit [Member]
Multiemployer Plans [Line Items]
Multiemployer plan, period contributions	$ 34,600us-gaap_MultiemployerPlanPeriodContributions / us-gaap_MultiemployerPlanTypeAxis = us-gaap_MultiemployerPlansPostretirementBenefitMember	$ 37,100us-gaap_MultiemployerPlanPeriodContributions / us-gaap_MultiemployerPlanTypeAxis = us-gaap_MultiemployerPlansPostretirementBenefitMember	$ 31,400us-gaap_MultiemployerPlanPeriodContributions / us-gaap_MultiemployerPlanTypeAxis = us-gaap_MultiemployerPlansPostretirementBenefitMember

X

- Definition

Amount of contributions made to multiemployer plans by the employer. A multiemployer plan is a pension or postretirement benefit plan to which two or more unrelated employers contribute where assets contributed by one participating employer may be used to provide benefits to employees of other participating employers.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 80

-Section 50

-Paragraph 11

-URI http://asc.fasb.org/extlink&oid=35742057&loc=SL14450702-114947



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 80

-Section 50

-Paragraph 9

-URI http://asc.fasb.org/extlink&oid=35742057&loc=SL14450691-114947



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 80

-Section 50

-Paragraph 5

-Subparagraph (e)(1)

-URI http://asc.fasb.org/extlink&oid=35742057&loc=SL14450657-114947

+ Details

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

X

- Details

Name:	us-gaap_MultiemployerPlansLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	us-gaap_DefinedContributionPlanNameAxis=mdu_MultiemployerPlansDefinedContributionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_MultiemployerPlanTypeAxis=us-gaap_MultiemployerPlansPostretirementBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: