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.