Condensed Statements of Income and Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Condensed Financial Statements, Captions [Line Items]
Operating revenues	$ 1,385,160	$ 1,587,289	$ 1,362,928	$ 1,197,373	$ 1,378,213	$ 1,563,799	$ 1,303,573	$ 1,091,191	$ 5,532,750	$ 5,336,776	$ 4,531,552
Operating expenses	1,239,270	1,383,578	1,224,674	1,140,303	1,247,992	1,374,329	1,206,262	1,026,973	4,987,825	4,855,556	4,129,829
Operating income	145,890	203,711	138,254	57,070	130,221	189,470	97,311	64,218	544,925	481,220	401,723
Other income									26,711	15,812	(238)
Interest expense									96,519	98,587	84,614
Income before income taxes									475,117	398,445	316,871
Income taxes									84,590	63,279	47,485
Net income from continuing operations	$ 112,131	$ 153,015	$ 99,842	$ 25,539	$ 94,804	$ 136,128	$ 63,145	$ 41,089	390,527	335,166	269,386
Comprehensive income									384,229	331,693	279,269
MDU Resources Group, Inc. [Member]
Condensed Financial Statements, Captions [Line Items]
Operating revenues									0	0	628,331
Operating expenses									0	0	540,125
Operating income									0	0	88,206
Other income									0	0	1,504
Interest expense									0	0	32,761
Income before income taxes									0	0	56,949
Income taxes									0	0	(4,259)
Equity in earnings of subsidiaries from continuing operations									390,527	335,166	208,177
Net income from continuing operations									390,527	335,166	269,385
Equity in earnings (loss) of subsidiaries from discontinued operations									(322)	287	2,933
Net income									390,205	335,453	272,318
Comprehensive income									$ 384,229	$ 331,693	$ 279,269