Common Stock (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Dividends declared per common share	$ 0.7350	$ 0.7150	$ 0.6950
Company's subsidiaries net assets restricted from being used to transfer funds to the Company	$ 1,600
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	$ 322
MDU Resources Group, Inc. [Member]
Shares of common stock reserved for original issuance under the Stock Purchase Plan and K-Plan (in shares)	13.9
Centennial [Member]
Maximum distributions to the company as a percentage of consolidated net income after taxes, excluding noncash write-downs, for the preceding fiscal year (in hundredths)	100.00%