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Condensed Statements of Income and Comprehensive Income (Details 2) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2015 | Sep. 30, 2015 | Jun. 30, 2015 | Mar. 31, 2015 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues | $ 1,016,099 | $ 1,208,567 | $ 1,043,948 | $ 860,214 | $ 1,016,826 | $ 1,198,342 | $ 938,039 | $ 860,845 | $ 4,128,828 | $ 4,014,052 | $ 4,115,073
Operating expenses | 904,613 | 1,061,883 | 954,983 | 798,229 | 934,896 | 1,070,514 | 878,330 | 810,537 | 3,719,708 | 3,694,277 | 3,787,891
Operating income | $ 111,486 | $ 146,684 | $ 88,965 | $ 61,985 | $ 81,930 | $ 127,828 | $ 59,709 | $ 50,308 | 409,120 | 319,775 | 327,182
Other income |  |  |  |  |  |  |  |  | 4,956 | 18,457 | 9,138
Interest expense |  |  |  |  |  |  |  |  | 87,848 | 91,179 | 86,871
Income before income taxes |  |  |  |  |  |  |  |  | 326,228 | 247,053 | 249,449
Income taxes |  |  |  |  |  |  |  |  | 93,132 | 70,664 | 64,422
Earnings (loss) on common stock |  |  |  |  |  |  |  |  | 63,748 | (623,120) | 297,548
Comprehensive income (loss) |  |  |  |  |  |  |  |  | 65,848 | (617,480) | 294,335
MDU Resources Group, Inc. [Member] |  |  |  |  |  |  |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 561,266 | 556,112 | 628,578
Operating expenses |  |  |  |  |  |  |  |  | 469,062 | 478,198 | 547,820
Operating income |  |  |  |  |  |  |  |  | 92,204 | 77,914 | 80,758
Other income |  |  |  |  |  |  |  |  | 1,491 | 8,318 | 5,271
Interest expense |  |  |  |  |  |  |  |  | 31,519 | 23,562 | 21,055
Income before income taxes |  |  |  |  |  |  |  |  | 62,176 | 62,670 | 64,974
Income taxes |  |  |  |  |  |  |  |  | 6,355 | 15,882 | 16,819
Equity in earnings of subsidiaries from continuing operations |  |  |  |  |  |  |  |  | 177,275 | 129,601 | 136,872
Net income from continuing operations |  |  |  |  |  |  |  |  | 233,096 | 176,389 | 185,027
Equity in earnings (loss) of subsidiaries from discontinued operations attributable to the company |  |  |  |  |  |  |  |  | (168,663) | (798,824) | 113,206
Dividends declared on preferred stocks |  |  |  |  |  |  |  |  | 685 | 685 | 685
Earnings (loss) on common stock |  |  |  |  |  |  |  |  | 63,748 | (623,120) | 297,548
Comprehensive income (loss) |  |  |  |  |  |  |  |  | $ 65,848 | $ (617,480) | $ 294,335

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