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Unpaid Claims and Claim Expenses - Narrative (Details) - USD ($) $ in Millions | 9 Months Ended
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Sep. 30, 2020 | Sep. 30, 2019
Liability for Catastrophe Claims [Line Items] |  |
Favorable development of total reserves for property and casualty claims occurring in prior years | $ 9.2 | $ 7.5
Automobile and homeowners loss |  |
Liability for Catastrophe Claims [Line Items] |  |
Favorable development of total reserves for property and casualty claims occurring in prior years | 4.0 |
2018 Camp Fire in California |  |
Liability for Catastrophe Claims [Line Items] |  |
Favorable development of total reserves for property and casualty claims occurring in prior years | $ 5.2 |

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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
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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_LiabilityForCatastropheClaimsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis=hmn_AutomobileAndHomeownersLossMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis=hmn_A2018CampFireInCaliforniaMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |