Condensed Statements of Income and Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Condensed Financial Statements, Captions [Line Items]
Operating revenues	$ 1,163,204	$ 1,370,455	$ 1,094,046	$ 1,042,853	$ 1,184,423	$ 1,285,782	$ 1,060,595	$ 931,604	$ 4,670,558	$ 4,462,404	$ 4,075,431
Operating expenses	1,053,582	1,193,690	995,112	939,949	1,037,306	1,135,909	969,217	827,073	4,182,333	3,969,505	4,056,224
Operating income	109,622	176,765	98,934	102,904	147,117	149,873	91,378	104,531	488,225	492,899	19,207
Other income									9,962	6,768	6,642
Interest expense									87,016	83,917	76,699
Income (loss) before income taxes									411,130	415,618	(45,467)
Income taxes									119,969	136,736	(31,146)
Net income (loss) attributable to parent	84,256	103,209	54,106	56,662	91,450	84,456	46,512	56,515
Earnings (loss) on common stock									297,548	278,248	(1,439)
Comprehensive income (loss)									294,335	289,449	(2,474)
MDU Resources Group, Inc. [Member]
Condensed Financial Statements, Captions [Line Items]
Operating revenues									628,578	549,239	472,302
Operating expenses									547,820	473,917	405,095
Operating income									80,758	75,322	67,207
Other income									5,271	3,709	3,925
Interest expense									21,055	17,386	17,297
Income (loss) before income taxes									64,974	61,645	53,835
Income taxes									16,819	13,520	11,798
Equity in earnings of subsidiaries									250,078	230,808	(42,791)
Net income (loss) attributable to parent									298,233	278,933	(754)
Dividends declared on preferred stocks									685	685	685
Earnings (loss) on common stock									297,548	278,248	(1,439)
Comprehensive income (loss)									$ 294,335	$ 289,449	$ (2,474)