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Pension Plans and Other Postretirement Benefits - Contributions (Details) $ in Thousands | 12 Months Ended
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Dec. 31, 2018 USD ($)
Defined Benefit Plan |
Defined Benefit Plan Disclosure [Line Items] |
Minimum funding requirement | $ 0
Expected contributions (approximations) | 0
Supplemental Defined Benefit Plans |
Defined Benefit Plan Disclosure [Line Items] |
Expected contributions (approximations) | $ 1,307

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- Definition Defined benefit plan and other post retirement plan expected contributions for the full next fiscal year as of the current fiscal year-end. + References No definition available. + Details
Name: | hmn_DefinedBenefitPlanAndOtherPostRetirementPlanExpectedContributionForNextFiscalYearEnd
Namespace Prefix: | hmn_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Defined benefit plan and other post retirement plan minimum funding required. + References No definition available. + Details
Name: | hmn_DefinedBenefitPlanAndOtherPostRetirementPlanMinimumFundingRequired
Namespace Prefix: | hmn_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- 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

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

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