Condensed Statements of Cash Flows (Details 4) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	$ 448,011	$ 462,209	$ 661,832
Capital expenditures	(341,382)	(388,183)	(536,832)
Net proceeds from sale or disposition of property and other	126,588	44,826	54,569
Investments	(1,608)	(1,396)	1,515
Net cash used in investing activities	(214,168)	(305,095)	(382,453)
Issuance of long-term debt	140,812	309,064	345,920
Repayment of long-term debt	(217,394)	(315,647)	(566,498)
Proceeds from issuance of common stock	0	0	21,898
Dividends paid	(150,727)	(147,156)	(142,835)
Redemption of preferred stock	(15,600)	0	0
Tax withholding on stock-based compensation	(757)	(323)	0
Net cash used in financing activities	(245,350)	(194,914)	(255,730)
Increase (decrease) in cash and cash equivalents	(11,508)	(37,796)	23,424
Cash and cash equivalents - beginning of year	46,107	83,903	60,479
Cash and cash equivalents - end of year	34,599	46,107	83,903
MDU Resources Group, Inc. [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	284,075	238,125	255,273
Capital expenditures	(146,370)	(159,570)	(349,985)
Net proceeds from sale or disposition of property and other	(5,665)	3,784	3,268
Investments in and advances to subsidiaries	(40,000)	(5,000)	(7,000)
Advances from subsidiaries	40,000	15,000	100,000
Investments	(468)	(129)	5
Net cash used in investing activities	(152,503)	(145,915)	(253,712)
Issuance of long-term debt	70,080	106,420	224,185
Repayment of long-term debt	(37,569)	(50,010)	(108,008)
Proceeds from issuance of common stock	0	0	21,898
Dividends paid	(150,727)	(147,156)	(142,835)
Redemption of preferred stock	(15,600)	0	0
Repurchase of common stock	(564)	0	0
Tax withholding on stock-based compensation	(508)	(226)	0
Net cash used in financing activities	(134,888)	(90,972)	(4,760)
Increase (decrease) in cash and cash equivalents	(3,316)	1,238	(3,199)
Cash and cash equivalents - beginning of year	4,159	2,921	6,120
Cash and cash equivalents - end of year	$ 843	$ 4,159	$ 2,921