Condensed Statements of Cash Flows (Details 3) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	$ 499,881	$ 448,011	$ 462,209
Capital expenditures	(568,230)	(341,382)	(388,183)
Net proceeds from sale or disposition of property and other	26,100	126,588	44,826
Investments	(2,321)	(1,608)	(1,396)
Net cash used in investing activities	(710,907)	(214,168)	(305,095)
Issuance of long-term debt	566,829	140,812	309,064
Repayment of long-term debt	(174,520)	(217,394)	(315,647)
Payments of stock issuance costs	(10)	0	0
Dividends paid	(154,573)	(150,727)	(147,156)
Redemption of preferred stock	0	(15,600)	0
Tax withholding on stock-based compensation	(2,330)	(757)	(323)
Net cash used in financing activities	230,376	(245,350)	(194,914)
Increase (decrease) in cash and cash equivalents	19,349	(11,508)	(37,796)
Cash and cash equivalents - beginning of year	34,599	46,107	83,903
Cash and cash equivalents - end of year	53,948	34,599	46,107
MDU Resources Group, Inc. [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	294,379	284,075	238,125
Capital expenditures	(242,692)	(146,370)	(159,570)
Net proceeds from sale or disposition of property and other	5,032	(5,665)	3,784
Investments in and advances to subsidiaries	(40,000)	(40,000)	(5,000)
Advances from subsidiaries	70,000	40,000	15,000
Investments	(528)	(468)	(129)
Net cash used in investing activities	(208,188)	(152,503)	(145,915)
Issuance of long-term debt	199,422	70,080	106,420
Repayment of long-term debt	(125,961)	(37,569)	(50,010)
Payments of stock issuance costs	(10)	0	0
Dividends paid	(154,573)	(150,727)	(147,156)
Redemption of preferred stock	0	(15,600)	0
Repurchase of common stock	(1,920)	(564)	0
Tax withholding on stock-based compensation	(1,721)	(508)	(226)
Net cash used in financing activities	(84,763)	(134,888)	(90,972)
Increase (decrease) in cash and cash equivalents	1,428	(3,316)	1,238
Cash and cash equivalents - beginning of year	843	4,159	2,921
Cash and cash equivalents - end of year	$ 2,271	$ 843	$ 4,159