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

Conditions resulting in an early termination date under any swap contract that is in excess of this amount

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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.

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