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Condensed Statements of Income and Comprehensive Income (Details 2) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues | $ 1,165,239 | $ 1,272,548 | $ 1,067,639 | $ 937,925 | $ 1,016,099 | $ 1,208,567 | $ 1,043,948 | $ 860,214 | $ 4,443,351 | $ 4,128,828 | $ 4,014,052
Operating expenses | 1,039,695 | 1,116,171 | 987,960 | 870,813 | 904,613 | 1,061,883 | 954,983 | 798,229 | 4,014,639 | 3,719,708 | 3,694,277
Operating income | $ 125,544 | $ 156,377 | $ 79,679 | $ 67,112 | $ 111,486 | $ 146,684 | $ 88,965 | $ 61,985 | 428,712 | 409,120 | 319,775
Other income |  |  |  |  |  |  |  |  | 4,103 | 4,956 | 18,457
Interest expense |  |  |  |  |  |  |  |  | 82,788 | 87,848 | 91,179
Income before income taxes |  |  |  |  |  |  |  |  | 350,027 | 326,228 | 247,053
Income taxes |  |  |  |  |  |  |  |  | 65,041 | 93,132 | 70,664
Loss on redemption of preferred stocks (Note 8) |  |  |  |  |  |  |  |  | 600 | 0 | 0
Earnings (loss) on common stock |  |  |  |  |  |  |  |  | 280,432 | 63,748 | (623,120)
Comprehensive income (loss) |  |  |  |  |  |  |  |  | 279,602 | 65,848 | (617,480)
MDU Resources Group, Inc. [Member] |  |  |  |  |  |  |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 623,693 | 561,266 | 556,112
Operating expenses |  |  |  |  |  |  |  |  | 516,524 | 469,062 | 478,198
Operating income |  |  |  |  |  |  |  |  | 107,169 | 92,204 | 77,914
Other income |  |  |  |  |  |  |  |  | 1,331 | 1,491 | 8,318
Interest expense |  |  |  |  |  |  |  |  | 31,997 | 31,519 | 23,562
Income before income taxes |  |  |  |  |  |  |  |  | 76,503 | 62,176 | 62,670
Income taxes |  |  |  |  |  |  |  |  | 13,800 | 6,355 | 15,882
Equity in earnings of subsidiaries from continuing operations |  |  |  |  |  |  |  |  | 222,283 | 177,275 | 129,601
Net income from continuing operations |  |  |  |  |  |  |  |  | 284,986 | 233,096 | 176,389
Equity in earnings (loss) of subsidiaries from discontinued operations attributable to the company |  |  |  |  |  |  |  |  | (3,783) | (168,663) | (798,824)
Loss on redemption of preferred stocks (Note 8) |  |  |  |  |  |  |  |  | 600 | 0 | 0
Dividends declared on preferred stocks |  |  |  |  |  |  |  |  | 171 | 685 | 685
Earnings (loss) on common stock |  |  |  |  |  |  |  |  | 280,432 | 63,748 | (623,120)
Comprehensive income (loss) |  |  |  |  |  |  |  |  | $ 279,602 | $ 65,848 | $ (617,480)

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