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v2.4.1.9

Condensed Statements of Income and Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013 | Mar. 31, 2013 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues | $ 1,163,204 us-gaap_Revenues | $ 1,370,455 us-gaap_Revenues | $ 1,094,046 us-gaap_Revenues | $ 1,042,853 us-gaap_Revenues | $ 1,184,423 us-gaap_Revenues | $ 1,285,782 us-gaap_Revenues | $ 1,060,595 us-gaap_Revenues | $ 931,604 us-gaap_Revenues | $ 4,670,558 us-gaap_Revenues | $ 4,462,404 us-gaap_Revenues | $ 4,075,431 us-gaap_Revenues
Operating expenses | 1,053,582 us-gaap_OperatingExpenses | 1,193,690 us-gaap_OperatingExpenses | 995,112 us-gaap_OperatingExpenses | 939,949 us-gaap_OperatingExpenses | 1,037,306 us-gaap_OperatingExpenses | 1,135,909 us-gaap_OperatingExpenses | 969,217 us-gaap_OperatingExpenses | 827,073 us-gaap_OperatingExpenses | 4,182,333 us-gaap_OperatingExpenses | 3,969,505 us-gaap_OperatingExpenses | 4,056,224 us-gaap_OperatingExpenses
Operating income | 109,622 us-gaap_OperatingIncomeLoss | 176,765 us-gaap_OperatingIncomeLoss | 98,934 us-gaap_OperatingIncomeLoss | 102,904 us-gaap_OperatingIncomeLoss | 147,117 us-gaap_OperatingIncomeLoss | 149,873 us-gaap_OperatingIncomeLoss | 91,378 us-gaap_OperatingIncomeLoss | 104,531 us-gaap_OperatingIncomeLoss | 488,225 us-gaap_OperatingIncomeLoss | 492,899 us-gaap_OperatingIncomeLoss | 19,207 us-gaap_OperatingIncomeLoss
Other income |  |  |  |  |  |  |  |  | 9,962 us-gaap_OtherNonoperatingIncomeExpense | 6,768 us-gaap_OtherNonoperatingIncomeExpense | 6,642 us-gaap_OtherNonoperatingIncomeExpense
Interest expense |  |  |  |  |  |  |  |  | 87,016 us-gaap_InterestExpense | 83,917 us-gaap_InterestExpense | 76,699 us-gaap_InterestExpense
Income (loss) before income taxes |  |  |  |  |  |  |  |  | 411,130 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest | 415,618 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest | (45,467)us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest
Income taxes |  |  |  |  |  |  |  |  | 119,969 us-gaap_IncomeTaxExpenseBenefit | 136,736 us-gaap_IncomeTaxExpenseBenefit | (31,146)us-gaap_IncomeTaxExpenseBenefit
Net income (loss) attributable to parent | 84,256 us-gaap_NetIncomeLoss | 103,209 us-gaap_NetIncomeLoss | 54,106 us-gaap_NetIncomeLoss | 56,662 us-gaap_NetIncomeLoss | 91,450 us-gaap_NetIncomeLoss | 84,456 us-gaap_NetIncomeLoss | 46,512 us-gaap_NetIncomeLoss | 56,515 us-gaap_NetIncomeLoss |  |  |
Earnings (loss) on common stock |  |  |  |  |  |  |  |  | 297,548 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | 278,248 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | (1,439)us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
Comprehensive income (loss) |  |  |  |  |  |  |  |  | 294,335 us-gaap_ComprehensiveIncomeNetOfTax | 289,449 us-gaap_ComprehensiveIncomeNetOfTax | (2,474)us-gaap_ComprehensiveIncomeNetOfTax
MDU Resources Group, Inc. [Member] |  |  |  |  |  |  |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 628,578 us-gaap_Revenues / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 549,239 us-gaap_Revenues / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 472,302 us-gaap_Revenues / dei_LegalEntityAxis = us-gaap_ParentCompanyMember
Operating expenses |  |  |  |  |  |  |  |  | 547,820 us-gaap_OperatingExpenses / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 473,917 us-gaap_OperatingExpenses / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 405,095 us-gaap_OperatingExpenses / dei_LegalEntityAxis = us-gaap_ParentCompanyMember
Operating income |  |  |  |  |  |  |  |  | 80,758 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 75,322 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 67,207 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = us-gaap_ParentCompanyMember
Other income |  |  |  |  |  |  |  |  | 5,271 us-gaap_OtherNonoperatingIncomeExpense / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 3,709 us-gaap_OtherNonoperatingIncomeExpense / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 3,925 us-gaap_OtherNonoperatingIncomeExpense / dei_LegalEntityAxis = us-gaap_ParentCompanyMember
Interest expense |  |  |  |  |  |  |  |  | 21,055 us-gaap_InterestExpense / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 17,386 us-gaap_InterestExpense / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 17,297 us-gaap_InterestExpense / dei_LegalEntityAxis = us-gaap_ParentCompanyMember
Income (loss) before income taxes |  |  |  |  |  |  |  |  | 64,974 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 61,645 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 53,835 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest / dei_LegalEntityAxis = us-gaap_ParentCompanyMember
Income taxes |  |  |  |  |  |  |  |  | 16,819 us-gaap_IncomeTaxExpenseBenefit / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 13,520 us-gaap_IncomeTaxExpenseBenefit / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 11,798 us-gaap_IncomeTaxExpenseBenefit / dei_LegalEntityAxis = us-gaap_ParentCompanyMember
Equity in earnings of subsidiaries |  |  |  |  |  |  |  |  | 250,078 mdu_EquityInEarningsOfSubsidiaries / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 230,808 mdu_EquityInEarningsOfSubsidiaries / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (42,791)mdu_EquityInEarningsOfSubsidiaries / dei_LegalEntityAxis = us-gaap_ParentCompanyMember
Net income (loss) attributable to parent |  |  |  |  |  |  |  |  | 298,233 us-gaap_NetIncomeLoss / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 278,933 us-gaap_NetIncomeLoss / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (754)us-gaap_NetIncomeLoss / dei_LegalEntityAxis = us-gaap_ParentCompanyMember
Dividends declared on preferred stocks |  |  |  |  |  |  |  |  | 685 us-gaap_DividendsPreferredStock / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 685 us-gaap_DividendsPreferredStock / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 685 us-gaap_DividendsPreferredStock / dei_LegalEntityAxis = us-gaap_ParentCompanyMember
Earnings (loss) on common stock |  |  |  |  |  |  |  |  | 297,548 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 278,248 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (1,439)us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / dei_LegalEntityAxis = us-gaap_ParentCompanyMember
Comprehensive income (loss) |  |  |  |  |  |  |  |  | $ 294,335 us-gaap_ComprehensiveIncomeNetOfTax / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | $ 289,449 us-gaap_ComprehensiveIncomeNetOfTax / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | $ (2,474)us-gaap_ComprehensiveIncomeNetOfTax / dei_LegalEntityAxis = us-gaap_ParentCompanyMember

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