Condensed Statements of Income and Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended												12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012	Dec. 31, 2011		Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Condensed Financial Statements, Captions [Line Items]
Operating revenues	$ 1,081,144		$ 1,173,518		$ 967,962		$ 852,807	$ 1,065,749		$ 1,152,181	$ 930,757	$ 901,805	$ 4,075,431	$ 4,050,492	$ 3,909,695
Operating expenses	1,190,673	[1]	1,207,553	[2]	876,248	[3]	781,750	939,172		1,032,760	848,454	823,739	4,056,224	3,644,125	3,498,968
Operating income	(109,529)	[1]	(34,035)	[2]	91,714	[3]	71,057	126,577		119,421	82,303	78,066	19,207	406,367	410,727
Other income													6,642	6,520	8,018
Interest expense													76,699	81,354	83,011
Income before income taxes													(45,467)	336,226	366,550
Income taxes													(31,146)	110,274	122,530
Net income (loss)	(60,986)	[1]	(29,671)	[2]	54,113	[3]	35,790	61,008	[1]	63,974	45,067	42,977	(754)	213,026	240,659
Dividends declared on preferred stocks													685	685	685
Earnings (loss) on common stock													(1,439)	212,341	239,974
Comprehensive income (loss)													(2,474)	197,286	230,231
MDU Resources Group, Inc. [Member]
Condensed Financial Statements, Captions [Line Items]
Operating revenues													472,302	518,268	503,658
Operating expenses													405,095	450,579	431,293
Operating income													67,207	67,689	72,365
Other income													3,925	2,710	5,734
Interest expense													17,297	18,660	16,664
Income before income taxes													53,835	51,739	61,435
Income taxes													11,798	10,476	17,983
Equity in earnings of subsidiaries													(42,791)	171,763	197,207
Net income (loss)													(754)	213,026	240,659
Dividends declared on preferred stocks													685	685	685
Earnings (loss) on common stock													(1,439)	212,341	239,974
Comprehensive income (loss)													$ (2,474)	$ 197,286	$ 230,231

[1] 2012 reflects a $145.9 million after-tax noncash write-down of oil and natural gas properties and the reversal of an arbitration charge of $13.0 million (after tax) related to a guarantee of a construction contract, which was partially offset by the reversal of estimated insurance recoveries, as previously discussed. 2011 reflects an arbitration charge of $13.0 million (after tax) related to a guarantee of a construction contract. For more information, see Notes 1 and 19, respectively.

[2] 2012 reflects a $100.9 million after-tax noncash write-down of oil and natural gas properties. For more information, see Note 1.

[3] 2012 reflects a net benefit of $15.0 million (after tax) related to natural gas gathering operations litigation and a net benefit largely related to estimated insurance recoveries related to the guarantee of a construction contract. For more information, see Note 19.