Condensed Balance Sheets (Details 2) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents	$ 53,948	$ 34,599	$ 46,107	$ 83,903
Receivables, net	722,945	727,030
Inventories	287,309	226,583
Prepayments and other current assets	119,500	81,304
Total current assets	1,184,132	1,069,995
Investments	138,620	137,613
Property, plant and equipment	7,397,321	6,770,829
Less accumulated depreciation, depletion and amortization	2,818,644	2,691,641	2,578,902
Net property, plant and equipment	4,578,677	4,079,188	3,931,327
Deferred charges and other assets:
Goodwill	664,922	631,791	631,791
Other	408,857	407,850
Total deferred charges and other assets	1,086,681	1,047,870
Total assets	6,988,110	6,334,666	6,284,467
Current liabilities:
Long-term debt due within one year	251,854	148,499
Accounts payable	358,505	312,327
Taxes payable	41,929	42,537
Dividends payable	39,695	38,573
Accrued compensation	69,007	72,919
Other accrued liabilities	221,059	186,010
Total current liabilities	986,050	812,858
Long-term debt	1,856,841	1,566,354
Deferred credits and other liabilities:
Deferred income taxes	430,085	347,271
Other	1,148,359	1,179,140
Total deferred credits and other liabilities	1,578,444	1,526,411
Commitments and contingencies
Commitments and contingencies
Authorized - 500,000,000 shares, $1.00 par value
Issued - 196,564,907 shares in 2018 and 195,843,297 shares in 2017	196,565	195,843
Other paid-in capital	1,248,576	1,233,412
Retained earnings	1,163,602	1,040,748
Accumulated other comprehensive loss	(38,342)	(37,334)
Treasury stock at cost - 538,921 shares	(3,626)	(3,626)
Total stockholders' equity	2,566,775	2,429,043
Total liabilities and stockholders' equity	$ 6,988,110	$ 6,334,666
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	196,564,907	195,843,297
Treasury stock, shares	538,921	538,921
MDU Resources Group, Inc. [Member]
Current assets:
Cash and cash equivalents	$ 2,271	$ 843	$ 4,159	$ 2,921
Receivables, net	92,724	83,453
Accounts receivable from subsidiaries	36,015	34,029
Inventories	13,293	13,864
Prepayments and other current assets	14,488	34,400
Total current assets	158,791	166,589
Investments	76,202	76,779
Investment in subsidiaries	1,790,886	1,704,908
Property, plant and equipment	2,846,715	2,631,161
Less accumulated depreciation, depletion and amortization	836,735	797,130
Net property, plant and equipment	2,009,980	1,834,031
Deferred charges and other assets:
Goodwill	4,812	4,812
Other	180,473	175,599
Total deferred charges and other assets	185,285	180,411
Total assets	4,221,144	3,962,718
Current liabilities:
Long-term debt due within one year	200,711	100,011
Accounts payable	50,051	47,000
Accounts payable to subsidiaries	12,438	7,234
Taxes payable	24,704	13,717
Dividends payable	39,695	38,573
Accrued compensation	14,346	20,017
Other accrued liabilities	54,099	36,881
Total current liabilities	396,044	263,433
Long-term debt	586,012	612,493
Deferred credits and other liabilities:
Deferred income taxes	165,122	147,847
Other	507,191	509,902
Total deferred credits and other liabilities	672,313	657,749
Commitments and contingencies
Commitments and contingencies
Authorized - 500,000,000 shares, $1.00 par value
Issued - 196,564,907 shares in 2018 and 195,843,297 shares in 2017	196,565	195,843
Other paid-in capital	1,248,576	1,233,412
Retained earnings	1,163,602	1,040,748
Accumulated other comprehensive loss	(38,342)	(37,334)
Treasury stock at cost - 538,921 shares	(3,626)	(3,626)
Total stockholders' equity	2,566,775	2,429,043
Total liabilities and stockholders' equity	$ 4,221,144	$ 3,962,718
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	196,564,907	195,843,297
Treasury stock, shares	538,921	538,921