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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, 2012 | Sep. 30, 2012 | Jun. 30, 2012 | Mar. 31, 2012 | Dec. 31, 2011 | Sep. 30, 2011 | Jun. 30, 2011 | Mar. 31, 2011 | Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Operating revenues | $ 1,081,144 |  | $ 1,173,518 |  | $ 967,962 |  | $ 852,807 | $ 1,065,749 |  | $ 1,152,181 | $ 930,757 | $ 901,805 | $ 4,075,431 | $ 4,050,492 | $ 3,909,695
Operating expenses | 1,190,673 | [1] | 1,207,553 | [2] | 876,248 | [3] | 781,750 | 939,172 |  | 1,032,760 | 848,454 | 823,739 | 4,056,224 | 3,644,125 | 3,498,968
Operating income | (109,529) | [1] | (34,035) | [2] | 91,714 | [3] | 71,057 | 126,577 |  | 119,421 | 82,303 | 78,066 | 19,207 | 406,367 | 410,727
Other income |  |  |  |  |  |  |  |  |  |  |  |  | 6,642 | 6,520 | 8,018
Interest expense |  |  |  |  |  |  |  |  |  |  |  |  | 76,699 | 81,354 | 83,011
Income before income taxes |  |  |  |  |  |  |  |  |  |  |  |  | (45,467) | 336,226 | 366,550
Income taxes |  |  |  |  |  |  |  |  |  |  |  |  | (31,146) | 110,274 | 122,530
Net income (loss) | (60,986) | [1] | (29,671) | [2] | 54,113 | [3] | 35,790 | 61,008 | [1] | 63,974 | 45,067 | 42,977 | (754) | 213,026 | 240,659
Dividends declared on preferred stocks |  |  |  |  |  |  |  |  |  |  |  |  | 685 | 685 | 685
Earnings (loss) on common stock |  |  |  |  |  |  |  |  |  |  |  |  | (1,439) | 212,341 | 239,974
Comprehensive income (loss) |  |  |  |  |  |  |  |  |  |  |  |  | (2,474) | 197,286 | 230,231
MDU Resources Group, Inc. [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  |  |  |  |  | 472,302 | 518,268 | 503,658
Operating expenses |  |  |  |  |  |  |  |  |  |  |  |  | 405,095 | 450,579 | 431,293
Operating income |  |  |  |  |  |  |  |  |  |  |  |  | 67,207 | 67,689 | 72,365
Other income |  |  |  |  |  |  |  |  |  |  |  |  | 3,925 | 2,710 | 5,734
Interest expense |  |  |  |  |  |  |  |  |  |  |  |  | 17,297 | 18,660 | 16,664
Income before income taxes |  |  |  |  |  |  |  |  |  |  |  |  | 53,835 | 51,739 | 61,435
Income taxes |  |  |  |  |  |  |  |  |  |  |  |  | 11,798 | 10,476 | 17,983
Equity in earnings of subsidiaries |  |  |  |  |  |  |  |  |  |  |  |  | (42,791) | 171,763 | 197,207
Net income (loss) |  |  |  |  |  |  |  |  |  |  |  |  | (754) | 213,026 | 240,659
Dividends declared on preferred stocks |  |  |  |  |  |  |  |  |  |  |  |  | 685 | 685 | 685
Earnings (loss) on common stock |  |  |  |  |  |  |  |  |  |  |  |  | (1,439) | 212,341 | 239,974
Comprehensive income (loss) |  |  |  |  |  |  |  |  |  |  |  |  | $ (2,474) | $ 197,286 | $ 230,231
[1] | 2012 reflects a $145.9 million after-tax noncash write-down of oil and natural gas properties and the reversal of an arbitration charge of $13.0 million (after tax) related to a guarantee of a construction contract, which was partially offset by the reversal of estimated insurance recoveries, as previously discussed. 2011 reflects an arbitration charge of $13.0 million (after tax) related to a guarantee of a construction contract. For more information, see Notes 1 and 19, respectively.
[2] | 2012 reflects a $100.9 million after-tax noncash write-down of oil and natural gas properties. For more information, see Note 1.
[3] | 2012 reflects a net benefit of $15.0 million (after tax) related to natural gas gathering operations litigation and a net benefit largely related to estimated insurance recoveries related to the guarantee of a construction contract. For more information, see Note 19.

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