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Condensed Statements of Income and Comprehensive Income (Details 2) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Condensed Financial Statements, Captions [Line Items]                      
Operating revenues $ 1,062,485 $ 1,280,500 $ 986,215 $ 862,349 $ 1,048,288 $ 1,213,203 $ 952,564 $ 900,761 $ 4,191,549 $ 4,114,816 $ 3,920,171
Operating expenses 1,007,092 1,170,757 940,887 818,680 973,720 1,094,310 890,210 837,153 3,937,416 3,795,393 3,599,469
Operating income $ 55,393 $ 109,743 $ 45,328 $ 43,669 $ 74,568 $ 118,893 $ 62,354 $ 63,608 254,133 319,423 320,702
Other income                 19,232 9,873 6,086
Interest expense                 93,068 86,906 83,803
Income before income taxes                 180,297 242,390 242,985
Income taxes                 65,603 63,227 74,294
Earnings (loss) on common stock                 (623,120) 297,548 278,248
Comprehensive income (loss)                 (617,480) 294,335 289,449
MDU Resources Group, Inc. [Member]                      
Condensed Financial Statements, Captions [Line Items]                      
Operating revenues                 556,112 628,578 549,239
Operating expenses                 478,198 547,820 473,917
Operating income                 77,914 80,758 75,322
Other income                 8,318 5,271 3,709
Interest expense                 23,562 21,055 17,386
Income before income taxes                 62,670 64,974 61,645
Income taxes                 15,882 16,819 13,520
Equity in earnings of subsidiaries from continuing operations                 103,162 134,903 120,929
Net income attributable to the Company from continuing operations                 149,950 183,058 169,054
Equity in earnings (loss) of subsidiaries from discontinued operations                 (772,385) 115,175 109,879
Dividends declared on preferred stocks                 685 685 685
Earnings (loss) on common stock                 (623,120) 297,548 278,248
Comprehensive income (loss)                 $ (617,480) $ 294,335 $ 289,449