Condensed Balance Sheets (Details 2) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Current assets:
Cash and cash equivalents	$ 49,042		$ 162,772		$ 222,074		$ 175,114
Receivables, net	678,123		646,251
Inventories	317,415		274,205
Deferred income taxes	22,846		40,407
Prepayments and other current assets	42,351		43,316
Total current assets	1,128,081		1,194,638
Long-term Investments	103,243		109,424
Property, plant and equipment	8,107,751		7,646,222
Less accumulated depreciation, depletion and amortization	3,608,912		3,361,208		3,103,323
Net property, plant and equipment	4,498,839		4,285,014		4,115,180
Deferred charges and other assets:
Goodwill	636,039	[1]	634,931	[1]	634,633	[1]
Other	299,160		311,275
Total deferred charges and other assets	952,328		967,049
Total assets	6,682,491		6,556,125		6,303,549
Current liabilities:
Long-term debt due within one year	134,108		139,267
Accounts payable	388,015		337,228
Taxes payable	46,475		70,176
Dividends payable	171		31,794
Accrued compensation	48,448		47,804
Other accrued liabilities	204,698		259,320
Total current liabilities	850,115		898,753
Long-term debt	1,610,867		1,285,411
Deferred Credits and Other Liabilities
Deferred income taxes	755,102		769,166
Other liabilities	818,159		827,228
Total deferred credits and other liabilities	1,573,261		1,596,394
Stockholders' equity:
Preferred stocks	15,000		15,000
Authorized - 500,000,000 shares, $1.00 par value
Issued - 189,369,450 shares in 2012 and 189,332,485 shares in 2011	189,369		189,332
Other paid-in capital	1,039,080		1,035,739
Retained earnings	1,457,146		1,586,123
Accumulated other comprehensive loss	(48,721)		(47,001)		(31,261)
Treasury stock at cost - 538,921 shares	(3,626)		(3,626)
Total common stockholders' equity	2,633,248		2,760,567		2,677,802		2,556,647
Total stockholders' equity	2,648,248		2,775,567
Total liabilities and stockholders' equity	6,682,491		6,556,125
MDU Resources Group, Inc. [Member]
Current assets:
Cash and cash equivalents	3,596		6,900		6,275		30,103
Receivables, net	89,238		67,761
Accounts receivable from subsidiaries	2,957		28,734
Inventories	41,469		42,596
Deferred income taxes	3,685		2
Prepayments and other current assets	9,120		12,154
Total current assets	150,065		158,147
Long-term Investments	52,123		47,835
Investment in subsidiaries	2,253,294		2,402,891
Property, plant and equipment	1,581,776		1,453,089
Less accumulated depreciation, depletion and amortization	621,623		605,510
Net property, plant and equipment	960,153		847,579
Deferred charges and other assets:
Goodwill	4,812		4,812
Other	155,483		166,732
Total deferred charges and other assets	160,295		171,544
Total assets	3,575,930		3,627,996
Current liabilities:
Long-term debt due within one year	108		107
Accounts payable	42,149		37,986
Accounts payable to subsidiaries	6,423		4,868
Taxes payable	12,399		18,304
Dividends payable	171		31,794
Accrued compensation	10,282		10,173
Other accrued liabilities	29,490		27,064
Total current liabilities	101,022		130,296
Long-term debt	356,760		280,781
Deferred Credits and Other Liabilities
Deferred income taxes	172,769		137,751
Other liabilities	297,131		303,601
Total deferred credits and other liabilities	469,900		441,352
Stockholders' equity:
Preferred stocks	15,000		15,000
Authorized - 500,000,000 shares, $1.00 par value
Issued - 189,369,450 shares in 2012 and 189,332,485 shares in 2011	189,369		189,332
Other paid-in capital	1,039,080		1,035,739
Retained earnings	1,457,146		1,586,123
Accumulated other comprehensive loss	(48,721)		(47,001)
Treasury stock at cost - 538,921 shares	(3,626)		(3,626)
Total common stockholders' equity	2,633,248		2,760,567
Total stockholders' equity	2,648,248		2,775,567
Total liabilities and stockholders' equity	$ 3,575,930		$ 3,627,996

[1] Balance is presented net of accumulated impairment of $12.3 million at the pipeline and energy services segment, which occurred in prior periods.