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Defined contribution plans (Details 12) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Nonqualified Defined Contribution Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, cost recognized	$ 104,000us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DefinedContributionPlanNameAxis = mdu_NonqualifiedDefinedContributionPlanMember	$ 25,000us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DefinedContributionPlanNameAxis = mdu_NonqualifiedDefinedContributionPlanMember
Defined Contribution Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, cost recognized	$ 34,400,000us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DefinedContributionPlanNameAxis = mdu_DefinedContributionPlanMember	$ 33,200,000us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DefinedContributionPlanNameAxis = mdu_DefinedContributionPlanMember	$ 29,300,000us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DefinedContributionPlanNameAxis = mdu_DefinedContributionPlanMember

X

- Definition

The amount of the cost recognized during the period for defined contribution plans.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 70

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6414718&loc=d3e28014-114942

+ Details

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Balance Type:	debit
Period Type:	duration

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- Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
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X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: