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Property and Casualty Unpaid Claims and Claim Expenses - Narrative (Details) - USD ($) $ in Thousands | 9 Months Ended
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Sep. 30, 2017 | Sep. 30, 2016
Insurance Loss Reserves [Abstract] |  |
Favorable development of total reserves for property and casualty claims occurring in prior years | $ 2,100 | $ 4,300

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