Condensed Statements of Cash Flows (Details 3) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	$ 584,740	$ 626,648	$ 551,633
Capital expenditures	(872,920)	(497,000)	(449,282)
Net proceeds from sale or disposition of property and other	40,110	40,107	76,386
Investments	9,725	(10,302)	704
Net cash used in investing activities	(887,952)	(464,545)	(407,944)
Issuance of short-term borrowings	20,100	0	20,000
Repayment of short-term borrowings	0	(20,000)	(10,300)
Issuance of long-term debt	467,957	300	20,200
Repayment of long-term debt	(138,775)	(85,151)	(13,668)
Proceeds from issuance of common stock	88	5,744	4,972
Dividends paid	(159,768)	(123,323)	(119,157)
Excess tax benefit on stock-based compensation	26	1,239	1,186
Net cash used in financing activities	189,628	(221,191)	(96,767)
Increase (decrease) in cash and cash equivalents	(113,730)	(59,302)	46,960
Cash and cash equivalents - beginning of year	162,772	222,074	175,114
Cash and cash equivalents - end of year	49,042	162,772	222,074
MDU Resources Group, Inc. [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	225,968	217,514	185,887
Capital expenditures	(150,337)	(74,580)	(114,045)
Net proceeds from sale or disposition of property and other	1,120	720	625
Investments in and advances to subsidiaries	(1,387)	(5,701)	(1,636)
Investments from and advances from subsidiaries	5,000	0	0
Investments	12	0	(742)
Net cash used in investing activities	(145,592)	(79,561)	(115,798)
Issuance of short-term borrowings	0	0	20,000
Repayment of short-term borrowings	0	(20,000)	0
Issuance of long-term debt	76,000	0	0
Repayment of long-term debt	(21)	(107)	(107)
Proceeds from issuance of common stock	88	5,744	4,972
Dividends paid	(159,768)	(123,323)	(119,157)
Excess tax benefit on stock-based compensation	21	358	375
Net cash used in financing activities	(83,680)	(137,328)	(93,917)
Increase (decrease) in cash and cash equivalents	(3,304)	625	(23,828)
Cash and cash equivalents - beginning of year	6,900	6,275	30,103
Cash and cash equivalents - end of year	$ 3,596	$ 6,900	$ 6,275