XML 82 R70.htm IDEA: XBRL DOCUMENT
v3.21.2

Fair value measurements (Details 3) - USD ($) $ in Thousands | Sep. 30, 2021 | Dec. 31, 2020 | Sep. 30, 2020
--- | --- | --- | ---
Concentration risks, percentage [Abstract] |  |  |
Percentage in fixed-income and other investments | 63.00% | 57.00% | 68.00%
Percentage investment in common stock of large-cap companies | 16.00% | 18.00% | 15.00%
Percentage investment in common stock of mid-cap companies | 8.00% | 9.00% | 6.00%
Percentage investment in common stock of small-cap companies | 7.00% | 9.00% | 6.00%
Percentage investment in target date investments | 5.00% | 5.00% | 4.00%
Percentage investment in cash and cash equivalents | 1.00% | 2.00% | 1.00%
Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | $ 127,610 | $ 120,348 | $ 115,356
Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 127,610 | 120,348 | 115,356
Money market funds | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 9,907 | 8,917 | 8,478
Money market funds | Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 9,907 | 8,917 | 8,478
Insurance contracts* | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 106,371 | 100,104 | 95,687
Insurance contracts* | Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 106,371 | 100,104 | 95,687
Mortgage-backed securities | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 8,679 | 9,946 | 10,022
Mortgage-backed securities | Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 8,679 | 9,946 | 10,022
U.S. Treasury securities | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 2,653 | 1,381 | 1,169
U.S. Treasury securities | Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | $ 2,653 | $ 1,381 | $ 1,169

X
- Definition Concentration Risks, Percentage + References No definition available. + Details
Name: | mdu_ConcentrationRisksPercentageAbstract
Namespace Prefix: | mdu_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The percentage of insurance contracts in fixed-income and other investments. + References No definition available. + Details
Name: | mdu_PercentageInFixedIncomeAndOtherInvestments
Namespace Prefix: | mdu_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition The percentage of insurance contracts in common stock of large-cap companies. + References No definition available. + Details
Name: | mdu_PercentageInvestmentInCommonStockOfLargeCapCompanies
Namespace Prefix: | mdu_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition The percentage of insurance contracts in common stock of mid-cap companies. + References No definition available. + Details
Name: | mdu_PercentageInvestmentInCommonStockOfMidCapCompanies
Namespace Prefix: | mdu_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition The percentage of insurance contracts in common stock of small-cap companies. + References No definition available. + Details
Name: | mdu_PercentageInvestmentInCommonStockOfSmallCapCompanies
Namespace Prefix: | mdu_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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

X
- Definition The percentage of insurance contracts in target date investments. + References No definition available. + Details
Name: | mdu_PercentageInvestmentinTargetDateInvestments
Namespace Prefix: | mdu_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
Name: | us-gaap_AssetsFairValueDisclosure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
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: |

X
- Details
Name: | us-gaap_FairValueByAssetClassAxis=us-gaap_MoneyMarketFundsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FairValueByAssetClassAxis=us-gaap_CashSurrenderValueMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FairValueByAssetClassAxis=us-gaap_MortgageBackedSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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