Notes to Condensed Financial Statements (Details 5) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Financial Statements, Captions [Line Items]
Long-term debt	$ 1,790,159,000	$ 1,796,163,000
2017	43,598,000
2018	169,449,000
2019	162,154,000
2020	15,021,000
Mature in years after 2021	1,254,860,000
MDU Resources Group, Inc. [Member]
Condensed Financial Statements, Captions [Line Items]
Long-term debt	681,800,000
2017	100,000
2018	100,100,000
2019	111,100,000
2020	100,000
Mature in years after 2021	470,400,000
Cash dividends paid to parent company by consolidated subsidiaries	$ 115,800,000	$ 110,600,000	$ 105,600,000