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Property and Casualty Unpaid Claims and Claim Expenses - Narrative (Details) - USD ($) | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Insurance Loss Reserves [Abstract] |  |  |
Automobile liability coverage percent reserves | 80.00% |  |
Short tail coverage reserves as a percentage of total reserves | 20.00% |  |
Long tail coverage reserves as percentage of reserves | 80.00% |  |
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 | $ 13,000.0 |  |
Favorable development of total reserves for property and casualty claims occurring in prior years | $ 300,000 | $ 2,700,000 | $ 7,000,000

X
- 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

X
- 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

X
- 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

X
- 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
Name: | hmn_PotentialVariabilityOfPropertyAndCasualtyLossReserves
Namespace Prefix: | hmn_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- 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

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

X
- References No definition available. + Details
Name: | us-gaap_InsuranceLossReservesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration