Condensed Balance Sheets (Details 3) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015		Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 34,599	$ 46,107	$ 83,903		$ 60,479
Receivables, net	727,030	630,243
Inventories	226,583	238,273
Prepayments and other current assets	81,304	48,461
Total current assets	1,069,995	977,475
Investments	137,613	125,866
Property, plant and equipment	6,770,829	6,510,229
Less accumulated depreciation, depletion and amortization	2,691,641	2,578,902	2,489,322
Net property, plant and equipment	4,079,188	3,931,327	3,898,380
Deferred charges and other assets:
Goodwill	631,791	631,791	635,204	[1]
Other	407,850	415,419
Total deferred charges and other assets	1,047,870	1,249,799
Total assets	6,334,666	6,284,467	6,565,154
Current liabilities:
Long-term debt due within one year	148,499	43,598
Accounts payable	312,327	279,962
Taxes payable	42,537	48,164
Dividends payable	38,573	37,767
Accrued compensation	72,919	65,867
Other accrued liabilities	186,010	184,377
Total current liabilities	812,858	669,659
Long-term debt	1,566,354	1,746,561
Deferred credits and other liabilities:
Deferred income taxes	347,271	668,226
Other	1,179,140	883,777
Total deferred credits and other liabilities	1,526,411	1,552,003
Commitments and contingencies
Equity:
Preferred stocks	0	15,000
Authorized - 500,000,000 shares, $1.00 par value
Issued - 195,843,297 shares in 2017 and 2016	195,843	195,843
Other paid-in capital	1,233,412	1,232,478
Retained earnings	1,040,748	912,282
Accumulated other comprehensive loss	(37,334)	(35,733)
Treasury stock at cost - 538,921 shares	(3,626)	(3,626)
Total common stockholders' equity	2,429,043	2,301,244
Total stockholders' equity	2,429,043	2,316,244
Total liabilities and stockholders' equity	$ 6,334,666	$ 6,284,467
Condensed Balance Sheet (Parenthetical)
Common stock, shares authorized	500,000,000	500,000,000
Common stock, par value	$ 1.00	$ 1.00
Common stock, shares issued	195,843,297	195,843,297
Treasury stock, shares	538,921	538,921
MDU Resources Group, Inc. [Member]
Current assets:
Cash and cash equivalents	$ 843	$ 4,159	$ 2,921		$ 6,120
Receivables, net	83,453	80,467
Accounts receivable from subsidiaries	34,029	34,424
Inventories	13,864	17,352
Prepayments and other current assets	34,400	24,531
Total current assets	166,589	160,933
Investments	76,779	70,370
Investment in subsidiaries	1,704,908	1,603,874
Property, plant and equipment	2,631,161	2,502,264
Less accumulated depreciation, depletion and amortization	797,130	756,191
Net property, plant and equipment	1,834,031	1,746,073
Deferred charges and other assets:
Goodwill	4,812	4,812
Other	175,599	183,654
Total deferred charges and other assets	180,411	188,466
Total assets	3,962,718	3,769,716
Current liabilities:
Long-term debt due within one year	100,011	110
Accounts payable	47,000	37,697
Accounts payable to subsidiaries	7,234	5,592
Taxes payable	13,717	14,992
Dividends payable	38,573	37,767
Accrued compensation	20,017	16,086
Other accrued liabilities	36,881	34,929
Total current liabilities	263,433	147,173
Long-term debt	612,493	679,667
Deferred credits and other liabilities:
Deferred income taxes	147,847	270,126
Other	509,902	356,506
Total deferred credits and other liabilities	657,749	626,632
Commitments and contingencies
Equity:
Preferred stocks	0	15,000
Authorized - 500,000,000 shares, $1.00 par value
Issued - 195,843,297 shares in 2017 and 2016	195,843	195,843
Other paid-in capital	1,233,412	1,232,478
Retained earnings	1,040,748	912,282
Accumulated other comprehensive loss	(37,334)	(35,733)
Treasury stock at cost - 538,921 shares	(3,626)	(3,626)
Total common stockholders' equity	2,429,043	2,301,244
Total stockholders' equity	2,429,043	2,316,244
Total liabilities and stockholders' equity	$ 3,962,718	$ 3,769,716
Condensed Balance Sheet (Parenthetical)
Common stock, shares authorized	500,000,000	500,000,000
Common stock, par value	$ 1.00	$ 1.00
Common stock, shares issued	195,843,297	195,843,297
Treasury stock, shares	538,921	538,921

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