Credit facilities (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Line of credit facility and other debt [Abstract]
Letters of credit at end of period	$ 25.0
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	76.0	[1]	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	217.0	[1]	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
Commercial paper program supported by revolving credit agreement with various banks	125
Revolving credit agreement	125
Option to increase borrowings, maximum amount	150
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	20.2	[2]
Commercial paper program supported by revolving credit agreement with various banks	500
Revolving credit agreement	500
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	2.0		0
Letters of credit at end of period	0
Expiration date	Dec. 27, 2013
Option to increase borrowings, maximum amount	75
Short-term debt [Member] | Revolving credit agreement [Member] | Intermountain Gas Company [Member]
Line of credit facility and other debt [Abstract]
Facility limit	65.0	[4]
Amount outstanding, end of period	26.2		8.1
Letters of credit at end of period	0
Expiration date	Aug. 11, 2013
Option to increase borrowings, maximum amount	$ 80

[1] Amount outstanding under commercial paper program

[2] The outstanding letters of credit, as discussed in Note 19, reduce amounts available under the credit agreement.

[3] Certain provisions allow for increased borrowings, up to a maximum of $75 million.

[4] Certain provisions allow for increased borrowings, up to a maximum of $80 million.