Common stock (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Dividends declared per common share	$ 0.7750	$ 0.7550	$ 0.7350
Company's subsidiaries net assets restricted from being used to transfer funds to the Company	$ 1,300
Credit agreement limitation on company ratio of funded debt to capitalization (excluding subsidiaries)	65.00%
Company's (excluding its subsidiaries) net assets restricted from use for dividend payments	$ 384
MDU Resources Group, Inc. [Member]
Shares of common stock reserved for original issuance under the K-Plan (in shares)	7.8
Centennial [Member]
Maximum distributions to the company as a ratio of average consolidated indebtedness to consolidated EBITDA	300.00%