Credit facilities (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		May 08, 2014	Dec. 31, 2013		Dec. 01, 2014
Line of credit facility and other debt [Abstract]
Letters of credit at end of period	$ 36.4
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, maximum borrowing capacity	175.0		175.0
Amount outstanding, end of period	77.5	[1]		78.9	[1]
Expiration date	May 08, 2019
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, maximum borrowing capacity	650.0		650.0
Amount outstanding, end of period	211.0	[1]		75.0	[1]
Expiration date	May 08, 2019
Long-term debt [Member] | Revolving credit facility [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	225.0
Long-term debt [Member] | Revolving credit facility [Member] | Cascade Natural Gas Corporation [Member]
Line of credit facility and other debt [Abstract]
Facility limit, maximum borrowing capacity	50.0	[2]
Amount outstanding, end of period	0			11.5
Letters of credit at end of period	2.2	[3]
Expiration date	Jul. 09, 2018
Option to increase borrowings, maximum amount	75.0
Long-term debt [Member] | Revolving credit facility [Member] | Intermountain Gas Company [Member]
Line of credit facility and other debt [Abstract]
Facility limit, maximum borrowing capacity	65.0	[4]
Amount outstanding, end of period	21.0			3.0
Letters of credit at end of period	0
Expiration date	Jul. 13, 2018
Option to increase borrowings, maximum amount	90.0
Long-term debt [Member] | Revolving credit facility [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	800.0
Short-term debt [Member] | Revolving credit facility [Member] | Dakota Prairie Refining, LLC [Member]
Line of credit facility and other debt [Abstract]
Facility limit, maximum borrowing capacity	50.0	[5]				50.0
Amount outstanding, end of period	0			0
Letters of credit at end of period	1.0	[3]
Expiration date	Dec. 01, 2015
Option to increase borrowings, maximum amount	$ 75.0

[1] Amount outstanding under commercial paper program.

[2] Certain provisions allow for increased borrowings, up to a maximum of $75.0 million.

[3] An outstanding letter of credit reduces the amount available under the credit agreement.

[4] Certain provisions allow for increased borrowings, up to a maximum of $90.0 million.

[5] Certain provisions allow for increased borrowings up to a maximum of $75.0 million.