Notes to Condensed Financial Statements (Details 5) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
2018	$ 148,499,000
2019	125,504,000
2020	73,012,000
2021	15,312,000
2022	147,214,000
Mature in years after 2022	1,211,007,000
MDU Resources Group, Inc. [Member]
Condensed Financial Statements, Captions [Line Items]
2018	100,000,000.0
2019	74,500,000
2020	700,000
2021	700,000
2022	700,000
Mature in years after 2022	538,100,000
Cash dividends paid to parent company by consolidated subsidiaries	$ 116,100,000	$ 115,800,000	$ 110,600,000