XML 128 R112.htm IDEA: XBRL DOCUMENT
v3.10.0.1

Fair value - pension (Details 8) - Qualified Plan [Member] - Underfunded Plan [Member] - Pension Benefits - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | --- | ---
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of net plan assets at end of year | $ 307,809 | $ 354,384 | $ 333,509
Percentage investment in common stock of international companies | 27.00% | 31.00% |
Percentage investment in corporate bonds | 31.00% | 28.00% |
Percentage investment in common stock of large-cap U.S. companies | 18.00% | 19.00% |
Percentage investment in cash equivalents | 5.00% | 7.00% |
Percentage Investment in US Government Securities |  | 1.00% |
Percentage investment in other investments | 19.00% | 14.00% |
Estimated future employer contributions in next fiscal year | $ 4,000 |  |
Cash equivalents [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of net plan assets at end of year | 4,930 | $ 3,814 |
Equity securities of U.S. companies [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of net plan assets at end of year | 11,038 | 13,345 |
Equities securities of International companies [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of net plan assets at end of year | 967 | 1,766 |
Collective and mutual funds [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of net plan assets at end of year | 197,560 | 239,571 |
Corporate debt securities [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of net plan assets at end of year | 73,110 | 74,956 |
Municipal bonds [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of net plan assets at end of year | 10,624 | 8,546 |
U.S. Government securities [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of net plan assets at end of year | 6,375 | 9,331 |
Total assets measured at fair value [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of net plan assets at end of year | 304,604 | 351,329 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity securities of U.S. companies [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of net plan assets at end of year | 11,038 | 13,345 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equities securities of International companies [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of net plan assets at end of year |  | 1,766 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Collective and mutual funds [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of net plan assets at end of year | 145,960 | 171,822 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Government securities [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of net plan assets at end of year | 479 | 1,038 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Total assets measured at fair value [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of net plan assets at end of year | 157,477 | 187,971 |
Significant Other Observable Inputs (Level 2) | Cash equivalents [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of net plan assets at end of year | 4,930 | 3,814 |
Significant Other Observable Inputs (Level 2) | Equities securities of International companies [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of net plan assets at end of year | 967 |  |
Significant Other Observable Inputs (Level 2) | Collective and mutual funds [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of net plan assets at end of year | 51,600 | 67,749 |
Significant Other Observable Inputs (Level 2) | Corporate debt securities [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of net plan assets at end of year | 73,110 | 74,956 |
Significant Other Observable Inputs (Level 2) | Municipal bonds [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of net plan assets at end of year | 10,624 | 8,546 |
Significant Other Observable Inputs (Level 2) | U.S. Government securities [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of net plan assets at end of year | 5,896 | 8,293 |
Significant Other Observable Inputs (Level 2) | Total assets measured at fair value [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of net plan assets at end of year | $ 147,127 | $ 163,358 |

X
- Definition Percentage investment in common stock of international companies. + References No definition available. + Details
Name: | mdu_PercentageInvestmentInCommonStockOfInternationalCompanies
Namespace Prefix: | mdu_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Percentage investment in common stock of large-cap U.S. companies. + References No definition available. + Details
Name: | mdu_PercentageInvestmentInCommonStockOfLargeCapUSCompanies
Namespace Prefix: | mdu_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Percentage investment in corporate bonds. + References No definition available. + Details
Name: | mdu_PercentageInvestmentInCorporateBonds
Namespace Prefix: | mdu_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Percentage investment in U.S. Government securities. + References No definition available. + Details
Name: | mdu_PercentageInvestmentinUSGovernmentSecurities
Namespace Prefix: | mdu_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Percentage investment in cash equivalents + References No definition available. + Details
Name: | mdu_Percentageinvestmentincashequivalents
Namespace Prefix: | mdu_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Percentage investment in other investments + References No definition available. + Details
Name: | mdu_Percentageinvestmentinotherinvestments
Namespace Prefix: | mdu_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_DefinedBenefitPlanDisclosureLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of contributions expected to be received by defined benefit plan from employer in next fiscal year following latest fiscal year. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
Name: | us-gaap_DefinedBenefitPlanExpectedFutureEmployerContributionsNextFiscalYear
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of asset segregated and restricted to provide benefit under defined benefit plan. Asset includes, but is not limited to, stock, bond, other investment, earning from investment, and contribution by employer and employee. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(01) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(01) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(ii) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 Reference 4: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=112275985&loc=d3e4179-114921 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 Reference 6: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(ii) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
Name: | us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_RetirementPlanTaxStatusAxis=us-gaap_QualifiedPlanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RetirementPlanFundingStatusAxis=us-gaap_UnderfundedPlanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RetirementPlanTypeAxis=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_CashAndCashEquivalentsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=mdu_EquitySecuritiesOfUSCompaniesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=mdu_EquitiesSecuritiesOfInternationalCompaniesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=mdu_CollectiveAndMutualFundsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_MunicipalBondsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_USTreasurySecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=mdu_TotalAssetsMeasuredatFairValueMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |