Notes to Condensed Financial Statements (Details 4) (MDU Resources Group, Inc. [Member], USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
MDU Resources Group, Inc. [Member]
Condensed Financial Statements, Captions [Line Items]
Long-term debt	$ 356,900,000
2013	100,000
2014	100,000
2015	100,000
2016	50,100,000
2017	76,000,000
Thereafter	230,500,000
Cash dividends paid to parent company by consolidated subsidiaries	$ 125,800,000	$ 96,100,000	$ 96,400,000