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Unpaid Claims and Claim Expenses - Narrative (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Segment Reporting Information [Line Items] |  |  |
Automobile liability coverage percent reserves | 74.00% |  |
Short tail coverage reserves as a percentage of total reserves | 26.00% |  |
Long tail coverage reserves as percentage of reserves | 74.00% |  |
Percentage of losses incurred | 99.00% |  |
Potential variability of property and casualty loss reserves | 6.00% |  |
Probability of other possible outcomes possible impact on net income | $ 12.0 |  |
Favorable development of total reserves for property and casualty claims occurring in prior years | 10.2 | $ 7.5 | $ 0.3
2018 Camp Fire In California |  |  |
Segment Reporting Information [Line Items] |  |  |
Favorable development of total reserves for property and casualty claims occurring in prior years | $ 4.8 |  |

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- Definition Automobile liability coverage reserves as a percentage of total reserves. + References No definition available. + Details
Name: | hmn_AutomobileLiabilityCoveragePercentReserves
Namespace Prefix: | hmn_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition The reduction in the estimate of the amount needed to settle claims relating to property and casualty insured events that have occurred in prior years. + References No definition available. + Details
Name: | hmn_FavorableDevelopmentOfTotalReservesForPropertyAndCasualtyClaimsOccurringInPriorYears
Namespace Prefix: | hmn_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Long tail coverage reserves as a percentage of total reserves. + References No definition available. + Details
Name: | hmn_LongTailCoverageReservesAsPercentageOfReserves
Namespace Prefix: | hmn_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition The percentage of losses represented by auto liability and auto physical damage. + References No definition available. + Details
Name: | hmn_PercentageOfLossesIncurred
Namespace Prefix: | hmn_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Potential percentage variability (plus or minus) of property and casualty loss reserves within a reasonable probability of other possible outcomes. + References No definition available. + Details
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Namespace Prefix: | hmn_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Equated Net Income impact (plus or minus) of potential percentage variability (plus or minus) of property and casualty loss reserves within a reasonable probability of other possible outcomes. + References No definition available. + Details
Name: | hmn_ProbabilityOfOtherPossibleOutcomesPossibleImpactOnNetIncome
Namespace Prefix: | hmn_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Short Tail Coverage Reserves As A Percentage Of Total Reserves. + References No definition available. + Details
Name: | hmn_ShortTailCoverageReservesAsAPercentageOfTotalReserves
Namespace Prefix: | hmn_
Data Type: | num:percentItemType
Balance Type: | na
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_SegmentReportingInformationLineItems
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Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |