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Stock based compensation plans (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 Y | Dec. 31, 2013 | Dec. 31, 2012
Share-based Compensation [Abstract] |  |  |
Share-based compensation arrangement, number of shares available for grant | 5.6 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant |  |
Share-based compensation expense, net of tax | $ 4.4 us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax | $ 3.9 us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax | $ 4.0 us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax
Share-based compensation nonvested awards total compensation cost not yet recognized | $ 8.0 mdu_ShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized |  |
Share-based compensation, nonvested awards, total compensation cost not yet recognized, period for recognition | 1.6 mdu_ShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition |  |

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- Definition As of the balance sheet date, the aggregate unrecognized cost of equity-based awards under equity-based compensation awards that have yet to vest. + References No definition available. + Details
Name: | mdu_ShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition The weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, using a decimal to express in number of years. + References No definition available. + Details
Name: | mdu_ShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition
Namespace Prefix: | mdu_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | instant

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- Definition The amount of expense, net of income tax, recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + References No definition available. + Details
Name: | us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
Name: | us-gaap_ShareBasedCompensationAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant