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Credit facilities (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Line of credit facility and other debt [Abstract]    
Letters of credit at end of period $ 36.0  
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 125.0  
Amount outstanding, end of period 78.9 [1] 76.0 [1]
Expiration date Oct. 04, 2017  
Long-term debt [Member] | Commercial paper revolving credit agreement [Member] | Centennial Energy Holdings, Inc [Member]
    
Line of credit facility and other debt [Abstract]    
Facility limit 500.0  
Amount outstanding, end of period 75.0 [1] 217.0 [1]
Expiration date Jun. 08, 2017  
Long-term debt [Member] | Revolving credit agreement [Member] | MDU Resources Group, Inc [Member]
    
Line of credit facility and other debt [Abstract]    
Letters of credit at end of period 0  
Option to increase borrowings, maximum amount 150  
Long-term debt [Member] | Revolving credit agreement [Member] | Intermountain Gas Company [Member]
    
Line of credit facility and other debt [Abstract]    
Facility limit 65.0 [2]  
Amount outstanding, end of period 3.0 26.2
Letters of credit at end of period 0  
Expiration date Jul. 13, 2018  
Option to increase borrowings, maximum amount 90  
Long-term debt [Member] | Revolving credit agreement [Member] | Centennial Energy Holdings, Inc [Member]
    
Line of credit facility and other debt [Abstract]    
Letters of credit at end of period 0  
Option to increase borrowings, maximum amount 650  
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 [3]  
Amount outstanding, end of period 11.5 2.0
Letters of credit at end of period 2.2 [4]  
Expiration date Jul. 09, 2018  
Option to increase borrowings, maximum amount $ 75  
[1] Amount outstanding under commercial paper program
[2] Certain provisions allow for increased borrowings, up to a maximum of $90 million.
[3] Certain provisions allow for increased borrowings, up to a maximum of $75 million.
[4] The outstanding letter of credit, as discussed in Note 19, reduces the amount available under the credit agreement.