Condensed Balance Sheets (Details 2) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 59,547	$ 66,459	$ 53,948	$ 34,599
Receivables, net	873,986	836,605
Prepayments and other current assets	44,120	52,617
Total current assets	1,337,347	1,297,701
Noncurrent Assets
Investments	165,022	148,656
Operating lease right-of-use assets	120,113	115,323
Other	144,111	153,849
Total noncurrent assets	6,716,025	6,385,358
Total assets	8,053,372	7,683,059	6,988,110
Current liabilities:
Accounts payable	426,264	403,391
Taxes payable	88,844	48,970
Dividends payable	42,611	41,580
Accrued compensation	90,629	99,269
Operating lease liabilities due within one year	33,655	31,664
Other accrued liabilities	198,514	182,078
Total current liabilities	963,522	866,427
Noncurrent liabilities:
Operating lease liabilities	86,868	83,742
Other	327,773	291,826
Total noncurrent liabilities	4,010,745	3,969,386
Commitments and contingencies
Commitments and Contingencies
Stockholders' equity:
Common stock	201,061	200,923
Other paid-in capital	1,371,385	1,355,404
Retained earnings	1,558,363	1,336,647
Accumulated other comprehensive loss	(48,078)	(42,102)
Treasury stock at cost - 538,921 shares	(3,626)	(3,626)
Stockholders' Equity Attributable to Parent	3,079,105	2,847,246
Total liabilities and stockholders' equity	$ 8,053,372	$ 7,683,059
Common Stock, Shares Authorized	500,000,000	500,000,000
Common Stock, Par or Stated Value Per Share	$ 1.00	$ 1.00
Common Stock, Shares, Issued	201,061,198	200,922,790
Treasury Stock, Common, Shares	538,921	538,921
MDU Resources Group, Inc. [Member]
Current assets:
Cash and cash equivalents	$ 8,781	$ 12,326	$ 2,271	$ 843
Receivables, net	4,865	4,727
Accounts receivable from subsidiaries	50,539	49,943
Prepayments and other current assets	1,612	501
Total current assets	65,797	67,497
Noncurrent Assets
Investments	52,000	46,294
Investment in subsidiaries	3,069,956	2,842,068
Deferred income taxes	9,691	7,269
Operating lease right-of-use assets	56	153
Other	28,866	27,098
Total noncurrent assets	3,160,569	2,922,882
Total assets	3,226,366	2,990,379
Current liabilities:
Accounts payable	2,135	2,981
Accounts payable to subsidiaries	5,412	4,752
Taxes payable	4,056	1,253
Dividends payable	42,611	41,580
Accrued compensation	7,825	8,812
Operating lease liabilities due within one year	40	96
Other accrued liabilities	6,881	7,690
Total current liabilities	68,960	67,164
Noncurrent liabilities:
Operating lease liabilities	16	56
Other	78,285	75,913
Total noncurrent liabilities	78,301	75,969
Commitments and contingencies
Commitments and Contingencies
Stockholders' equity:
Common stock	201,061	200,923
Other paid-in capital	1,371,385	1,355,404
Retained earnings	1,558,363	1,336,647
Accumulated other comprehensive loss	(48,078)	(42,102)
Treasury stock at cost - 538,921 shares	(3,626)	(3,626)
Stockholders' Equity Attributable to Parent	3,079,105	2,847,246
Total liabilities and stockholders' equity	$ 3,226,366	$ 2,990,379
Common Stock, Shares Authorized	500,000,000	500,000,000
Common Stock, Par or Stated Value Per Share	$ 1.00	$ 1.00
Common Stock, Shares, Issued	201,061,198	200,922,790
Treasury Stock, Common, Shares	538,921	538,921