Condensed Statements of Income and Comprehensive Income (Details 2) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Operating revenues	$ 1,165,239	$ 1,272,548	$ 1,067,639	$ 937,925	$ 1,016,099	$ 1,208,567	$ 1,043,948	$ 860,214	$ 4,443,351	$ 4,128,828	$ 4,014,052
Operating expenses	1,039,695	1,116,171	987,960	870,813	904,613	1,061,883	954,983	798,229	4,014,639	3,719,708	3,694,277
Operating income	$ 125,544	$ 156,377	$ 79,679	$ 67,112	$ 111,486	$ 146,684	$ 88,965	$ 61,985	428,712	409,120	319,775
Other income									4,103	4,956	18,457
Interest expense									82,788	87,848	91,179
Income before income taxes									350,027	326,228	247,053
Income taxes									65,041	93,132	70,664
Loss on redemption of preferred stocks (Note 8)									600	0	0
Earnings (loss) on common stock									280,432	63,748	(623,120)
Comprehensive income (loss)									279,602	65,848	(617,480)
MDU Resources Group, Inc. [Member]
Condensed Financial Statements, Captions [Line Items]
Operating revenues									623,693	561,266	556,112
Operating expenses									516,524	469,062	478,198
Operating income									107,169	92,204	77,914
Other income									1,331	1,491	8,318
Interest expense									31,997	31,519	23,562
Income before income taxes									76,503	62,176	62,670
Income taxes									13,800	6,355	15,882
Equity in earnings of subsidiaries from continuing operations									222,283	177,275	129,601
Net income from continuing operations									284,986	233,096	176,389
Equity in earnings (loss) of subsidiaries from discontinued operations attributable to the company									(3,783)	(168,663)	(798,824)
Loss on redemption of preferred stocks (Note 8)									600	0	0
Dividends declared on preferred stocks									171	685	685
Earnings (loss) on common stock									280,432	63,748	(623,120)
Comprehensive income (loss)									$ 279,602	$ 65,848	$ (617,480)