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Condensed Statements of Income and Comprehensive Income (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 12 Months Ended
Dec. 31, 2013
Sep. 30, 2013
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2012
Sep. 30, 2012
Jun. 30, 2012
Mar. 31, 2012
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Condensed Financial Statements, Captions [Line Items]                      
Operating revenues $ 1,184,423 $ 1,285,782 $ 1,060,595 $ 931,604 $ 1,081,144 $ 1,173,518 $ 967,962 $ 852,807 $ 4,462,404 $ 4,075,431 $ 4,050,492
Operating expenses 1,037,306 1,135,909 969,217 827,073 1,190,673 1,207,553 876,248 781,750 3,969,505 4,056,224 3,644,125
Operating income 147,117 149,873 91,378 104,531 (109,529) (34,035) 91,714 71,057 492,899 19,207 406,367
Other income                 6,768 6,642 6,520
Interest expense                 83,917 76,699 81,354
Income (loss) before income taxes                 415,618 (45,467) 336,226
Income taxes                 136,736 (31,146) 110,274
Net income (loss) attributable to parent 91,450 84,456 46,512 56,515 (60,986) (29,671) 54,113 35,790      
Earnings (loss) on common stock                 278,248 (1,439) 212,341
Comprehensive income (loss)                 289,449 (2,474) 197,286
MDU Resources Group, Inc. [Member]
                      
Condensed Financial Statements, Captions [Line Items]                      
Operating revenues                 549,239 472,302 518,268
Operating expenses                 473,917 405,095 450,579
Operating income                 75,322 67,207 67,689
Other income                 3,709 3,925 2,710
Interest expense                 17,386 17,297 18,660
Income (loss) before income taxes                 61,645 53,835 51,739
Income taxes                 13,520 11,798 10,476
Equity in earnings of subsidiaries                 230,808 (42,791) 171,763
Net income (loss) attributable to parent                 278,933 (754) 213,026
Dividends declared on preferred stocks                 685 685 685
Earnings (loss) on common stock                 278,248 (1,439) 212,341
Comprehensive income (loss)                 $ 289,449 $ (2,474) $ 197,286