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Asset Retirement Obligations (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Asset retirement obligation [Roll Forward]
Balance, beginning of period	$ 98,529,000us-gaap_AssetRetirementObligation	$ 102,545,000us-gaap_AssetRetirementObligation
Liabilities incurred	5,416,000us-gaap_AssetRetirementObligationLiabilitiesIncurred	5,610,000us-gaap_AssetRetirementObligationLiabilitiesIncurred
Liabilities acquired	1,414,000mdu_AssetRetirementObligationLiabilitiesAcquired	0mdu_AssetRetirementObligationLiabilitiesAcquired
Asset retirement obligation, liabilities settled	(18,388,000)us-gaap_AssetRetirementObligationLiabilitiesSettled	(22,257,000)us-gaap_AssetRetirementObligationLiabilitiesSettled
Accretion expense	4,605,000us-gaap_AssetRetirementObligationAccretionExpense	4,574,000us-gaap_AssetRetirementObligationAccretionExpense
Revisions in estimates	884,000us-gaap_AssetRetirementObligationRevisionOfEstimate	7,671,000us-gaap_AssetRetirementObligationRevisionOfEstimate
Other	379,000mdu_AssetRetirementObligationOther	386,000mdu_AssetRetirementObligationOther
Balance, end of period	92,839,000us-gaap_AssetRetirementObligation	98,529,000us-gaap_AssetRetirementObligation
Fair value of assets legally restricted for purposes of settling asset retirement obligations	$ 2,300,000us-gaap_AssetRetirementObligationLegallyRestrictedAssetsFairValue	$ 4,100,000us-gaap_AssetRetirementObligationLegallyRestrictedAssetsFairValue

X

- Definition

Amount of asset retirement obligations liabilities acquired during the period.

+ References

No definition available.

+ Details

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

X

- Definition

Amount of the Increase or Decrease in the amount of asset retirement obligations for the period associated with other obligations.

+ References

No definition available.

+ Details

Name:	mdu_AssetRetirementObligationOther
Namespace Prefix:	mdu_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 410

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849



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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Asset Retirement Obligation

-URI http://asc.fasb.org/extlink&oid=6505190

+ Details

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

X

- Definition

Amount of accretion expense recognized during the period that is associated with an asset retirement obligation. Accretion expense measures and incorporates changes due to the passage of time into the carrying amount of the liability.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 410

-SubTopic 20

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6392676&loc=d3e7480-110848



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 410

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (c)(3)

-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

+ Details

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

X

- Definition

Carrying amount of an asset that is legally restricted for purposes of settling an asset retirement obligation.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 410

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

+ Details

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Namespace Prefix:	us-gaap_
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X

- Definition

Amount of asset retirement obligations incurred during the period.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 410

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

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Balance Type:	credit
Period Type:	duration

X

- Definition

Amount of asset retirement obligations settled, or otherwise disposed of, during the period. This may include asset retirement obligations transferred to third parties associated with the sale of a long-lived asset.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 410

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (c)(2)

-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	debit
Period Type:	duration

X

- Definition

Amount of the Increase or Decrease in the amount of the asset retirement obligation during the current period for changes in the amount or timing of the estimated cash flows associated with the settlement of the obligation.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 410

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (c)(4)

-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

+ Details

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