Credit facilities (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014	Dec. 01, 2014
Line of credit facility and other debt [Abstract]
Letters of credit at end of period		$ 57.3
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
Amount outstanding, end of period	[1]	$ 44.5		$ 77.5
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
Amount outstanding, end of period	[1]	$ 18.0		211.0
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		$ 0.0
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	[2]	50.0
Amount outstanding, end of period		0.0		0.0
Letters of credit at end of period	[3]	$ 2.2
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	[4]	65.0
Amount outstanding, end of period		47.9		21.0
Letters of credit at end of period		$ 0.0
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		39.4
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		75.0	$ 75.0		$ 50.0
Amount outstanding, end of period		45.5		$ 0.0
Letters of credit at end of period	[3]	$ 18.3
Expiration date		Jun. 30, 2016

[1] Amount outstanding under commercial paper program.

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

[3] Outstanding letter(s) of credit reduce the amount available under the credit agreement.

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