Credit facilities (Details) - USD ($)		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015
Line of credit facility and other debt [Abstract]
Letters of credit at end of period		$ 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, maximum borrowing capacity		175,000,000
Amount outstanding, end of period	[1]	$ 111,000,000	$ 44,500,000
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		$ 500,000,000
Amount outstanding, end of period	[1]	$ 151,000,000	18,000,000
Expiration date		Sep. 23, 2021
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		225,000,000
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]	50,000,000
Amount outstanding, end of period		0	0
Letters of credit at end of period	[3]	$ 2,200,000
Expiration date		Jul. 09, 2018
Option to increase borrowings, maximum amount		$ 75,000,000
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]	65,000,000
Amount outstanding, end of period		20,900,000	$ 47,900,000
Letters of credit at end of period		$ 0
Expiration date		Jul. 13, 2018
Option to increase borrowings, maximum amount		$ 90,000,000
Long-term debt [Member] | Revolving credit facility [Member] | Centennial Energy Holdings, Inc [Member]
Line of credit facility and other debt [Abstract]
Facility limit, maximum borrowing capacity		500,000,000
Letters of credit at end of period		0
Option to increase borrowings, maximum amount		$ 600,000,000

[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.