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Long-term debt (Details) (USD $) In Millions, unless otherwise specified							0 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Jul. 15, 2014 The Company [Member]	May 08, 2014 The Company [Member]	Apr. 15, 2014 The Company [Member]	Jan. 28, 2014 The Company [Member]	Apr. 02, 2014 Centennial Energy Holdings, Inc [Member]	Mar. 31, 2014 Centennial Energy Holdings, Inc [Member]	May 08, 2014 Centennial Energy Holdings, Inc [Member]
Line of Credit Facility [Line Items]
Note purchase agreement						$ 150.0
Senior notes issued			100.0		50.0
Senior notes weighted average interest rate			4.30%		5.20%
Facility, maximum borrowing capacity				175.0					650.0
Ratio of total debt to total capitalization as specified in debt convenants							65.00%	65.00%	65.00%
Debt instrument, term							2 years	2 years
Term loan agreement							125.0	125.0
Long-term commercial paper	$ 293.5	$ 153.9

X

- Definition

Note purchase agreement.

+ References

No definition available.

+ Details

Name:	mdu_NotePurchaseAgreement
Namespace Prefix:	mdu_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

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

+ References

No definition available.

+ Details

Name:	mdu_RatioOfTotalDebtToTotalCapitalizationAsSpecifiedInDebtConvenants
Namespace Prefix:	mdu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Senior notes issued.

+ References

No definition available.

+ Details

Name:	mdu_SeniorNotesIssued
Namespace Prefix:	mdu_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Weighted average interest rate of senior notes issued.

+ References

No definition available.

+ Details

Name:	mdu_SeniorNotesWeightedAverageInterestRate
Namespace Prefix:	mdu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Term loan agreement.

+ References

No definition available.

+ Details

Name:	mdu_Termloanagreement
Namespace Prefix:	mdu_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Details

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

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

X

- Definition

Including the current and noncurrent portions, carrying value as of the balance sheet date of long-term unsecured obligations issued by corporations and other borrowers to investors (with maturities initially due after one year or beyond the operating cycle if longer).

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 20

-Article 5



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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19

-Article 5



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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5



Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.16)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878



Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.16)

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910



Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19,20,22)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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