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Employee Benefit Plans Multiemployer Pension Plan (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2009
Multiemployer Plans [Line Items] |  |  |
Multiemployer plan red zone status funded percentage, maximum | 65.00% |  |
Multiemployer plan yellow zone status funded percentage, minimum | 65.00% |  |
Mulitiemployer plan yellow status funded percentage, maximum | 80.00% |  |
Multiemployer plan green zone status funded percentage, minimum | 80.00% |  |
Multiemployer Plans, Pension [Member] |  |  |
Multiemployer Plans [Line Items] |  |  |
Multiemployer Plan, Period Contributions | $ 29,158 | $ 30,276 | $ 32,479
Multiemployer Plans, Postretirement Benefit [Member] |  |  |
Multiemployer Plans [Line Items] |  |  |
Multiemployer Plan, Period Contributions | 24,000 | 24,700 | 28,900
Multiemployer Plans, Defined Contribution [Member] |  |  |
Multiemployer Plans [Line Items] |  |  |
Multiemployer Plan, Period Contributions | 15,300 | 15,400 | 16,400
Edison Pension Plan [Member] |  |  |
Multiemployer Plans [Line Items] |  |  |
Multiemployer Plans, Surcharge | no |  |
Multiemployer Plans, Collective-Bargaining Arrangement, Expiration Date | Dec. 31, 2012 |  |
Edison Pension Plan [Member] | Multiemployer Plans, Pension [Member] |  |  |
Multiemployer Plans [Line Items] |  |  |
Entity Tax Identification Number | 936061681 |  |
Multiemployer Plan Number | 001 |  |
Multiemployer Plans, Certified Zone Status | green | green |
Multiemployer Plans, Funding Improvement Plan and Rehabilitation Plan | no |  |
Multiemployer Plan, Period Contributions | 2,700 | 1,933 | 1,627
IBEW Local 38 Pension Plan [Member] |  |  |
Multiemployer Plans [Line Items] |  |  |
Multiemployer Plans, Surcharge | no |  |
IBEW Local 38 Pension Plan [Member] | Multiemployer Plans, Pension [Member] |  |  |
Multiemployer Plans [Line Items] |  |  |
Entity Tax Identification Number | 346574238 |  |
Multiemployer Plan Number | 001 |  |
Multiemployer Plans, Certified Zone Status | yellow | yellow |
Multiemployer Plans, Certified Zone Status, Date | Apr. 30, 2011 | Apr. 30, 2010 |
Multiemployer Plans, Funding Improvement Plan and Rehabilitation Plan | implemented |  |
Multiemployer Plan, Period Contributions | 1,469 | 1,277 | 594
IBEW Local No. 82 Pension Plan [Member] |  |  |
Multiemployer Plans [Line Items] |  |  |
Multiemployer Plans, Surcharge | no |  |
IBEW Local No. 82 Pension Plan [Member] | Multiemployer Plans, Pension [Member] |  |  |
Multiemployer Plans [Line Items] |  |  |
Entity Tax Identification Number | 316127268 |  |
Multiemployer Plan Number | 001 |  |
Multiemployer Plans, Certified Zone Status | red | red |
Multiemployer Plans, Certified Zone Status, Date | Jun. 30, 2011 | Jun. 30, 2010 |
Multiemployer Plans, Funding Improvement Plan and Rehabilitation Plan | implemented |  |
Multiemployer Plan, Period Contributions | 1,331 | 1,569 | 1,197
IBEW Local 648 Pension Plan [Member] |  |  |
Multiemployer Plans [Line Items] |  |  |
Multiemployer Plans, Surcharge | no |  |
Multiemployer Plans, Collective-Bargaining Arrangement, Expiration Date | Aug. 31, 2012 |  |
IBEW Local 648 Pension Plan [Member] | Multiemployer Plans, Pension [Member] |  |  |
Multiemployer Plans [Line Items] |  |  |
Entity Tax Identification Number | 316134845 |  |
Multiemployer Plan Number | 001 |  |
Multiemployer Plans, Certified Zone Status | red | red |
Multiemployer Plans, Certified Zone Status, Date | Feb. 28, 2011 | Feb. 28, 2010 |
Multiemployer Plans, Funding Improvement Plan and Rehabilitation Plan | implemented |  |
Multiemployer Plan, Period Contributions | 722 | 781 | 641
Laborers Pension Trust Fund for Northern California [Member] |  |  |
Multiemployer Plans [Line Items] |  |  |
Multiemployer Plans, Surcharge | no |  |
Multiemployer Plans, Collective-Bargaining Arrangement, Expiration Date | Jun. 30, 2012 |  |
Laborers Pension Trust Fund for Northern California [Member] | Multiemployer Plans, Pension [Member] |  |  |
Multiemployer Plans [Line Items] |  |  |
Entity Tax Identification Number | 946277608 |  |
Multiemployer Plan Number | 001 |  |
Multiemployer Plans, Certified Zone Status | yellow | yellow |
Multiemployer Plans, Certified Zone Status, Date | May 31, 2011 | May 31, 2010 |
Multiemployer Plans, Funding Improvement Plan and Rehabilitation Plan | implemented |  |
Multiemployer Plan, Period Contributions | 628 | 413 | 325
Local Union 212 IBEW Pension Trust Fund [Member] |  |  |
Multiemployer Plans [Line Items] |  |  |
Multiemployer Plans, Surcharge | no |  |
Local Union 212 IBEW Pension Trust Fund [Member] | Multiemployer Plans, Pension [Member] |  |  |
Multiemployer Plans [Line Items] |  |  |
Entity Tax Identification Number | 316127280 |  |
Multiemployer Plan Number | 001 |  |
Multiemployer Plans, Certified Zone Status | yellow | yellow |
Multiemployer Plans, Certified Zone Status, Date | Apr. 30, 2011 | Apr. 30, 2010 |
Multiemployer Plans, Funding Improvement Plan and Rehabilitation Plan | implemented |  |
Multiemployer Plan, Period Contributions | 776 | 679 | 469
National Electrical Benefit Fund [Member] |  |  |
Multiemployer Plans [Line Items] |  |  |
Multiemployer Plans, Surcharge | no |  |
Multiemployer Plans, Collective-Bargaining Arrangement, Expiration Date | May 31, 2014 |  |
National Electrical Benefit Fund [Member] | Multiemployer Plans, Pension [Member] |  |  |
Multiemployer Plans [Line Items] |  |  |
Entity Tax Identification Number | 530181657 |  |
Multiemployer Plan Number | 001 |  |
Multiemployer Plans, Certified Zone Status | green | green |
Multiemployer Plans, Funding Improvement Plan and Rehabilitation Plan | no |  |
Multiemployer Plan, Period Contributions | 4,841 | 4,826 | 5,462
OE Pension Trust Fund [Member] |  |  |
Multiemployer Plans [Line Items] |  |  |
Multiemployer Plans, Surcharge | no |  |
Multiemployer Plans, Collective-Bargaining Arrangement, Expiration Date | Mar. 31, 2016 |  |
OE Pension Trust Fund [Member] | Multiemployer Plans, Pension [Member] |  |  |
Multiemployer Plans [Line Items] |  |  |
Entity Tax Identification Number | 946090764 |  |
Multiemployer Plan Number | 001 |  |
Multiemployer Plans, Certified Zone Status | yellow | yellow |
Multiemployer Plans, Funding Improvement Plan and Rehabilitation Plan | implemented |  |
Multiemployer Plan, Period Contributions | 1,367 | 1,035 | 1,061
Multiemployer Plan, Individually Insignificant Multiemployer Plans [Member] | Multiemployer Plans, Pension [Member] |  |  |
Multiemployer Plans [Line Items] |  |  |
Multiemployer Plan, Period Contributions | $ 15,324 | $ 17,763 | $ 21,103

X
- Definition The Tax Identification Number (TIN), also known as an Employer Identification Number (EIN), is a unique 9-digit value assigned by the IRS. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation 12B -Number 240 -Section 12b -Subsection 1 + Details
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- Definition Mulitiemployer plan yellow status funded percentage, maximum. + References No definition available. + Details
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- Definition Multiemployer plan green zone status funded percentage, minimum. + References No definition available. + Details
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- Definition Three digit plan number for a pension or postretirement benefit plan to which two or more unrelated employers contribute where assets contributed by one participating employer may be used to provide benefits to employees of other participating employers. + References No definition available. + Details
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Balance Type: | na
Period Type: | duration

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- Definition Multiemployer plan red zone status funded percentage, maximum. + References No definition available. + Details
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- Definition Multiemployer Plans, Certified Zone Status + References No definition available. + Details
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- Definition Multiemployer Plans, Certified Zone Status, Date + References No definition available. + Details
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- Definition Multiemployer Plans, Collective-Bargaining Arrangement, Expiration Date + References No definition available. + Details
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Period Type: | duration

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- Definition Indicate whether a funding improvement plan or rehabilitation plan has been implemented or is pending for a pension or postretirement benefit plan to which two or more unrelated employers contribute where assets contributed by one participating employer may be used to provide benefits to employees of other participating employers. The acceptable values are "no", "pending", "implemented", "other", and "NA". + References No definition available. + Details
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- Details
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- Definition Multiemployer Plans, Surcharge + References No definition available. + Details
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- Definition Multiemployer plan yellow zone status funded percentage, minimum. + References No definition available. + Details
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Namespace Prefix: | mdu_
Data Type: | num:percentItemType
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X
- Definition Amount of contributions made to multiemployer plans for each period for which a statement of income is presented. An employer may disclose total contributions to multiemployer plans without disaggregating the amounts attributable to pension plans and other postretirement benefit plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 12 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Section 50 -Paragraph 1 -SubTopic 80 -Topic 715 -URI http://asc.fasb.org/extlink&oid=6958064&loc=d3e29142-114947 + Details
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