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Long-term Debt (Details) (USD $) | Dec. 31, 2011 | Dec. 31, 2010
--- | --- | ---
Long term debt outstanding [Abstract] |  |
Net long-term debt | $ 1,285,411,000 | $ 1,433,955,000
Senior Notes [Member] |  |
Long term debt outstanding [Abstract] |  |
Weighted average interest rate (in hundredths) | 6.01% |
Medium-term Notes [Member] |  |
Long term debt outstanding [Abstract] |  |
Weighted average interest rate (in hundredths) | 7.72% |
Other Notes [Member] |  |
Long term debt outstanding [Abstract] |  |
Weighted average interest rate (in hundredths) | 5.24% |
Loans Payable [Member] |  |
Long term debt outstanding [Abstract] |  |
Weighted average interest rate (in hundredths) | 2.98% |
MDU Resources Group, Inc [Member] |  |
Long term debt outstanding [Abstract] |  |
Long-term Debt | 280,900,000 |
Net long-term debt | 280,781,000 | 280,889,000
Long Term Debt Maturities [Abstract] |  |
2012 | 100,000 |
2013 | 100,000 |
2014 | 100,000 |
2015 | 100,000 |
2016 | 50,000,000 |
Thereafter 2016 | 230,500,000 |
Long-term Debt [Member] |  |
Long term debt outstanding [Abstract] |  |
Long-term Debt | 1,424,678,000 | 1,506,752,000
Less current maturities | 139,267,000 | 72,797,000
Net long-term debt | 1,285,411,000 | 1,433,955,000
Long Term Debt Maturities [Abstract] |  |
2012 | 139,300,000 |
2013 | 267,300,000 |
2014 | 9,300,000 |
2015 | 266,400,000 |
2016 | 288,400,000 |
Thereafter 2016 | 454,000,000 |
Long-term Debt [Member] | Senior Notes [Member] |  |
Long term debt outstanding [Abstract] |  |
Long-term Debt | 1,287,576,000 | 1,358,848,000
Long-term Debt [Member] | Medium-term Notes [Member] |  |
Long term debt outstanding [Abstract] |  |
Long-term Debt | 81,000,000 | 81,000,000
Long-term Debt [Member] | Other Notes [Member] |  |
Long term debt outstanding [Abstract] |  |
Long-term Debt | 40,469,000 | 41,189,000
Long-term Debt [Member] | Loans Payable [Member] |  |
Long term debt outstanding [Abstract] |  |
Long-term Debt | 15,633,000 | 25,715,000
Long-term Debt [Member] | Master Shelf Agreement Member [Member] | MDU Energy Capital, LLC [Member] |  |
Long-term Debt Covenants [Abstract] |  |
Ratio of total debt to adjusted total capitalization as specified in debt convenants | 70.00% |
Ratio of subsidiary debt to subsidiary capitalization as specified in debt convenants | 65.00% |
Ratio of total debt to total capitalization as specified in debt convenants. | 65.00% |
Long-term Debt [Member] | Uncommitted Long Term Private Shelf Agreement [Member] | Williston Basin Interstate Pipeline Company [Member] |  |
Long-term Debt Covenants [Abstract] |  |
Ratio of total debt to total capitalization as specified in debt convenants. | 55.00% |
Long-term Debt [Member] | Revolving Credit Agreement [Member] | MDU Resources Group, Inc [Member] |  |
Long-term Debt Covenants [Abstract] |  |
Ratio of funded debt to total capitalization as specified in debt convenants | 65.00% |
Ratio of funded debt capitalization - Company alone, as specified in debt convenants | 65.00% |
Long-term Debt [Member] | Revolving Credit Agreement [Member] | Intermountain Gas Company [Member] |  |
Long-term Debt Covenants [Abstract] |  |
Ratio of funded debt to total capitalization as specified in debt convenants | 65.00% |
Conditions resulting in an early termination date under any swap contract that is in excess of this amount | $ 10,000,000 |

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- Definition Including current and noncurrent portions, aggregate carrying amount of long-term borrowings as of the balance sheet date. May include notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt, which had initial maturities beyond one year or beyond the normal operating cycle, if longer, and after deducting unamortized discount or premiums, if any. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 7 -Section 03 -Paragraph 16 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 9 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 20, 22 -Article 5 + Details
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- Definition Total of the portions of the carrying amounts as of the balance sheet date of long-term debt, which may include notes payable, bonds payable, debentures, mortgage loans, and commercial paper, which are scheduled to be repaid within one year or the normal operating cycle, if longer, and after deducting unamortized discount or premiums, if any. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 20 -Article 5 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19 -Article 5 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 210.5-02.19,20) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Amount of long-term debt maturities after year five following the date of the latest balance sheet presented in the financial statements, which may include maturities of long-term debt, sinking fund requirements, and other securities redeemable at fixed of determinable prices and dates. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 47 -Paragraph 10 -Subparagraph b -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 -Topic 470 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601 + Details
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