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Subsquent events (Details) (Subsequent event [Member], USD $) In Millions, unless otherwise specified	0 Months Ended
	Oct. 01, 2013 MDU Energy Capital [Member]	Oct. 30, 2013 Cascade Natural Gas Co. [Member]	Oct. 30, 2013 Intermountain Gas Company [Member]	Nov. 01, 2013 Centennial Energy Holdings, Inc [Member]
Subsequent Event [Line Items]
Credit agreement	$ 30.0
Senior notes issued		$ 50.0	$ 50.0	$ 25.0
Senior notes weighted average interest rate		4.24%	4.21%	4.84%

X

- Definition

Credit agreement.

+ References

No definition available.

+ Details

Name:	mdu_CreditAgreement
Namespace Prefix:	mdu_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Describes the event or transaction that occurred between the balance sheet date and the date the financial statements are issued or available to be issued.

+ References

No definition available.

+ Details

Name:	mdu_SeniorNotesIssued
Namespace Prefix:	mdu_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Describes the event or transaction that occurred between the balance sheet date and the date the financial statements are issued or available to be issued.

+ References

No definition available.

+ Details

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

X

- Details

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