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Credit Facilities (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended |
--- | --- | ---
Dec. 31, 2011 | Dec. 31, 2010
Line of Credit Facility and Other Debt [Abstract] |  |  |  |
Letters of Credit at end of period | $ 27.4 |  |  |
Long-term Debt [Member] | Commercial Paper Revolving Credit Agreement [Member] | MDU Resources Group, Inc [Member] |  |  |  |
Line of Credit Facility and Other Debt [Abstract] |  |  |  |
Facility Limit | 100.0 |  |  |
Amount outstanding, end of period | 0 | [1] | 20.0 | [2]
Letters of Credit at end of period | 0 |  |  |
Expiration Date | 5/26/2015 |  |  |
Commercial paper program supported by revolving credit agreement with various banks | 125 |  |  |
Long-term Debt [Member] | Revolving Credit Agreement [Member] | MDU Resources Group, Inc [Member] |  |  |  |
Line of Credit Facility and Other Debt [Abstract] |  |  |  |
Revolving Credit Agreement | 100 |  |  |
Option to increase borrowings, maximum amount | 150 |  |  |
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] |  |  |  |
Line of Credit Facility and Other Debt [Abstract] |  |  |  |
Facility Limit | 65.0 | [3] |  |
Amount outstanding, end of period | 8.1 |  | 20.2 |
Letters of Credit at end of period | 0 |  |  |
Expiration Date | 8/11/2013 |  |  |
Option to increase borrowings, maximum amount | 80 |  |  |
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 |  |  |
Long-term Debt [Member] | Master Shelf Agreement Member [Member] | MDU Energy Capital, LLC [Member] |  |  |  |
Line of Credit Facility and Other Debt [Abstract] |  |  |  |
Ratio of total debt to total capitalization as specified in debt convenants. | 65.00% |  |  |
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 EBIT to Interest Expense | 150.00% |  |  |
Long-term Debt [Member] | Master Shelf Agreement Member [Member] | Centennial Energy Holdings, Inc [Member] |  |  |  |
Line of Credit Facility and Other Debt [Abstract] |  |  |  |
Ratio of total debt to total capitalization as specified in debt convenants. | 60.00% |  |  |
Ratio of EBITDA to Interest Expense | 175.00% |  |  |
Short-term Debt [Member] | Commercial Paper Revolving Credit Agreement [Member] | Centennial Energy Holdings, Inc [Member] |  |  |  |
Line of Credit Facility and Other Debt [Abstract] |  |  |  |
Facility Limit | 400.0 |  |  |
Amount outstanding, end of period | 0 | [1] | 0 | [1]
Letters of Credit at end of period | 21.6 | [4] |  |
Expiration Date | 12/13/2012 |  |  |
Commercial paper program supported by revolving credit agreement with various banks | 400 |  |  |
Short-term Debt [Member] | Revolving Credit Agreement [Member] | Cascade Natural Gas Corporation [Member] |  |  |  |
Line of Credit Facility and Other Debt [Abstract] |  |  |  |
Facility Limit | 50.0 | [5] |  |
Amount outstanding, end of period | 0 |  | 0 |
Letters of Credit at end of period | 1.9 | [4] |  |
Expiration Date | 12/28/2012 |  |  |
Revolving Credit Agreement | 50 |  |  |
Option to increase borrowings, maximum amount | 75 |  |  |
Option to extend term of loan facility, maximum (in years) | two years |  |  |
Ratio of total debt to total capitalization as specified in debt convenants. | 65.00% |  |  |
Short-term Debt [Member] | Revolving Credit Agreement [Member] | Centennial Energy Holdings, Inc [Member] |  |  |  |
Line of Credit Facility and Other Debt [Abstract] |  |  |  |
Revolving Credit Agreement | 400 |  |  |
Option to increase borrowings, maximum amount | $ 450 |  |  |
Ratio of total debt to total capitalization as specified in debt convenants. | 65.00% |  |  |
[1] | Amount outstanding under commercial paper program.
[2] | Amount outstanding under commercial paper program that was classified as short-term borrowings because the revolving credit agreement expired within one year.
[3] | Certain provisions allow for increased borrowings, up to a maximum of $80 million.
[4] | The outstanding letters of credit, as discussed in Note 19, reduce amounts available under the credit agreement.
[5] | Certain provisions allow for increased borrowings, up to a maximum of $75 million.

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- Definition Option to Extend Rerm of Loan Facility, Period, Maximum + References No definition available. + Details
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