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Accounts receivable and allowance for doubtful accounts (Details) (USD $) In Millions | Sep. 30, 2011 | Dec. 31, 2010 | Sep. 30, 2010
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Accounts receivable and allowance for doubtful accounts [Abstract] |  |  |
Total balance of receivables past due 90 days or more | $ 27.9 | $ 21.6 |
Allowance for doubtful accounts | $ 12.1 | $ 15.3 | $ 15.9

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- Definition Amount due from customers or clients, within one year of the balance sheet date (or the normal operating cycle, whichever is longer), for goods or services (including trade receivables) that have been delivered or sold in the normal course of business, which are past due 90 days or more. + Details
Name: | mdu_ReceivablesPastDue90DaysOrMore
Namespace Prefix: | mdu
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition A valuation allowance for trade and other receivables due to an Entity within one year (or the normal operating cycle, whichever is longer) that are expected to be uncollectible. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 4 -Article 5 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6965416&loc=d3e5074-111524 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.4) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent
Namespace Prefix: | us-gaap
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
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Namespace Prefix: | us-gaap
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration