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Goodwill and other intangible assets (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Goodwill [Roll Forward]
Balance as of beginning of period	$ 636,039us-gaap_Goodwill	[1]	$ 636,039us-gaap_Goodwill	[1]
Goodwill acquired during the year/other	(835)us-gaap_GoodwillOtherChanges
Goodwill acquired during the year			0us-gaap_GoodwillAcquiredDuringPeriod
Balance as of end of period	635,204us-gaap_Goodwill	[1]	636,039us-gaap_Goodwill	[1]
Natural gas distribution:
Goodwill [Roll Forward]
Balance as of beginning of period	345,736us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = mdu_NaturalGasDistributionMember		345,736us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = mdu_NaturalGasDistributionMember
Goodwill acquired during the year/other	0us-gaap_GoodwillOtherChanges / us-gaap_StatementBusinessSegmentsAxis = mdu_NaturalGasDistributionMember
Goodwill acquired during the year			0us-gaap_GoodwillAcquiredDuringPeriod / us-gaap_StatementBusinessSegmentsAxis = mdu_NaturalGasDistributionMember
Balance as of end of period	345,736us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = mdu_NaturalGasDistributionMember		345,736us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = mdu_NaturalGasDistributionMember
Pipeline and energy services:
Goodwill [Roll Forward]
Balance as of beginning of period	9,737us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = mdu_PipelineAndEnergyServicesMember	[1]	9,737us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = mdu_PipelineAndEnergyServicesMember	[1]
Goodwill acquired during the year/other	0us-gaap_GoodwillOtherChanges / us-gaap_StatementBusinessSegmentsAxis = mdu_PipelineAndEnergyServicesMember
Goodwill acquired during the year			0us-gaap_GoodwillAcquiredDuringPeriod / us-gaap_StatementBusinessSegmentsAxis = mdu_PipelineAndEnergyServicesMember
Balance as of end of period	9,737us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = mdu_PipelineAndEnergyServicesMember	[1]	9,737us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = mdu_PipelineAndEnergyServicesMember	[1]
Accumulated impairment which occurred in prior periods	12,300us-gaap_GoodwillImpairedAccumulatedImpairmentLoss / us-gaap_StatementBusinessSegmentsAxis = mdu_PipelineAndEnergyServicesMember		12,300us-gaap_GoodwillImpairedAccumulatedImpairmentLoss / us-gaap_StatementBusinessSegmentsAxis = mdu_PipelineAndEnergyServicesMember
Construction materials and contracting:
Goodwill [Roll Forward]
Balance as of beginning of period	176,290us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionMaterialsAndContractingMember		176,290us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionMaterialsAndContractingMember
Goodwill acquired during the year/other	0us-gaap_GoodwillOtherChanges / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionMaterialsAndContractingMember
Goodwill acquired during the year			0us-gaap_GoodwillAcquiredDuringPeriod / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionMaterialsAndContractingMember
Balance as of end of period	176,290us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionMaterialsAndContractingMember		176,290us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionMaterialsAndContractingMember
Construction services:
Goodwill [Roll Forward]
Balance as of beginning of period	104,276us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionServicesMember		104,276us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionServicesMember
Goodwill acquired during the year/other	(835)us-gaap_GoodwillOtherChanges / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionServicesMember
Goodwill acquired during the year			0us-gaap_GoodwillAcquiredDuringPeriod / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionServicesMember
Balance as of end of period	$ 103,441us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionServicesMember		$ 104,276us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionServicesMember
[1] Balance is presented net of accumulated impairment of $12.3 million at the pipeline and energy services segment, which occurred in prior periods.

X

- Definition

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266

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X

- Definition

Amount of increase in asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized resulting from a business combination.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

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X

- Definition

Amount of accumulated impairment loss for an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (a,h)

-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

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- Definition

Amount of other (increase) decrease of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (g)

-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

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