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

X

- 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

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X

- 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

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X

- 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

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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X

- Details

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X

- 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

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