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Fair Value of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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U.S. government and federally sponsored agency obligations |  |  |  |
Investments, Carrying Value | $ 7,403,467 us-gaap_Investments |  | $ 6,539,473 us-gaap_Investments |
Investments, Fair Value | 149,792 us-gaap_InvestmentsFairValueDisclosure |  | 144,921 us-gaap_InvestmentsFairValueDisclosure |
Separate Account (variable annuity) assets, Carrying Value | 1,813,557 us-gaap_SeparateAccountAssets |  | 1,747,995 us-gaap_SeparateAccountAssets |
Recurring [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Carrying Amount | 6,893,090 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  | 6,009,573 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
Fair Value | 6,893,090 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  | 6,009,573 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
Investments, Carrying Value | 7,158,242 us-gaap_Investments / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  | 6,313,189 us-gaap_Investments / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
Investments, Fair Value | 7,158,242 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  | 6,313,189 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
Separate Account (variable annuity) assets, Carrying Value | 1,813,557 us-gaap_SeparateAccountAssets / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | [1] | 1,747,995 us-gaap_SeparateAccountAssets / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | [1]
Separate Account (variable annuity) assets, Fair Value | 1,813,557 hmn_SeparateAccountAssetsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | [1] | 1,747,995 hmn_SeparateAccountAssetsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | [1]
Financial Liabilities Carrying Amount | 20,049 hmn_LiabilitiesCarryingAmount / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  | 0 hmn_LiabilitiesCarryingAmount / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
Financial Liabilities Fair value | 20,049 us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  | 0 us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
Short-term investments [Member] | Recurring [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Investments, Carrying Value | 142,039 us-gaap_Investments / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember |  | 206,758 us-gaap_Investments / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember |
Investments, Fair Value | 142,039 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember |  | 206,758 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember |
Mortgage-backed securities [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Fair Value | 535,726 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember |  | 569,725 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember |
Mortgage-backed securities [Member] | Recurring [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Carrying Amount | 535,726 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember |  | 569,725 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember |
Other including U.S. Treasury Securities [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Fair Value | 538,199 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = hmn_OtherIncludingUsTreasurySecuritiesMember |  | 435,574 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = hmn_OtherIncludingUsTreasurySecuritiesMember |
Other including U.S. Treasury Securities [Member] | Recurring [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Carrying Amount | 538,199 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = hmn_OtherIncludingUsTreasurySecuritiesMember |  | 435,574 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = hmn_OtherIncludingUsTreasurySecuritiesMember |
Municipal Bonds [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Fair Value | 1,647,822 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember |  | 1,471,527 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember |
Municipal Bonds [Member] | Recurring [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Carrying Amount | 1,647,822 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember |  | 1,471,527 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember |
Foreign government bonds [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Fair Value | 59,536 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForeignGovernmentDebtMember |  | 54,951 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForeignGovernmentDebtMember |
Foreign government bonds [Member] | Recurring [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Carrying Amount | 59,536 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForeignGovernmentDebtMember |  | 54,951 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForeignGovernmentDebtMember |
Corporate Bonds [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Fair Value | 2,834,749 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  | 2,614,409 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |
Corporate Bonds [Member] | Recurring [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Carrying Amount | 2,834,749 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  | 2,614,409 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |
Other Mortgage-backed Securities [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Fair Value | 1,277,058 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesOtherMember |  | 863,387 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesOtherMember |
Other Mortgage-backed Securities [Member] | Recurring [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Carrying Amount | 1,277,058 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesOtherMember |  | 863,387 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesOtherMember |
Equity Securities [Member] | Recurring [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Carrying Amount | 110,655 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |  | 91,858 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |
Fair Value | 110,655 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |  | 91,858 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |
Other investments | Recurring [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Investments, Carrying Value | 12,458 us-gaap_Investments / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = hmn_OtherPolicyholdersFundsEmbeddedDerivativesMember |  | 5,000 us-gaap_Investments / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = hmn_OtherPolicyholdersFundsEmbeddedDerivativesMember |
Investments, Fair Value | 12,458 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = hmn_OtherPolicyholdersFundsEmbeddedDerivativesMember |  | 5,000 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = hmn_OtherPolicyholdersFundsEmbeddedDerivativesMember |
Level 1 [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Investments, Fair Value | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |  | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |
Level 1 [Member] | Recurring [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Fair Value | 28,381 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  | 27,938 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
Investments, Fair Value | 262,560 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  | 308,571 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
Separate Account (variable annuity) assets, Fair Value | 1,813,557 hmn_SeparateAccountAssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | [1] | 1,747,995 hmn_SeparateAccountAssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | [1]
Financial Liabilities Fair value | 0 us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  | 0 us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
Level 1 [Member] | Short-term investments [Member] | Recurring [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Investments, Fair Value | 142,039 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember |  | 206,354 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember |
Level 1 [Member] | Mortgage-backed securities [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Fair Value | 0 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember |  | 0 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember |
Level 1 [Member] | Other including U.S. Treasury Securities [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Fair Value | 17,857 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = hmn_OtherIncludingUsTreasurySecuritiesMember |  | 17,757 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = hmn_OtherIncludingUsTreasurySecuritiesMember |
Level 1 [Member] | Municipal Bonds [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Fair Value | 0 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember |  | 0 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember |
Level 1 [Member] | Foreign government bonds [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Fair Value | 0 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForeignGovernmentDebtMember |  | 0 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForeignGovernmentDebtMember |
Level 1 [Member] | Corporate Bonds [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Fair Value | 10,524 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  | 10,181 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |
Level 1 [Member] | Other Mortgage-backed Securities [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Fair Value | 0 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesOtherMember |  | 0 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesOtherMember |
Level 1 [Member] | Equity Securities [Member] | Recurring [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Fair Value | 92,140 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |  | 74,279 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |
Level 1 [Member] | Other investments | Recurring [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Investments, Fair Value | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = hmn_OtherPolicyholdersFundsEmbeddedDerivativesMember |  | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = hmn_OtherPolicyholdersFundsEmbeddedDerivativesMember |
Level 2 [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Investments, Fair Value | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |  | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |
Level 2 [Member] | Recurring [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Fair Value | 6,693,415 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  | 5,872,106 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
Investments, Fair Value | 6,724,382 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  | 5,895,083 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
Separate Account (variable annuity) assets, Fair Value | 0 hmn_SeparateAccountAssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | [1] | 0 hmn_SeparateAccountAssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | [1]
Financial Liabilities Fair value | 0 us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  | 0 us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
Level 2 [Member] | Short-term investments [Member] | Recurring [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Investments, Fair Value | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember |  | 404 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember |
Level 2 [Member] | Mortgage-backed securities [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Fair Value | 535,726 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember |  | 569,725 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember |
Level 2 [Member] | Other including U.S. Treasury Securities [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Fair Value | 520,342 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = hmn_OtherIncludingUsTreasurySecuritiesMember |  | 417,817 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = hmn_OtherIncludingUsTreasurySecuritiesMember |
Level 2 [Member] | Municipal Bonds [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Fair Value | 1,634,194 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember |  | 1,468,833 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember |
Level 2 [Member] | Foreign government bonds [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Fair Value | 59,536 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForeignGovernmentDebtMember |  | 54,951 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForeignGovernmentDebtMember |
Level 2 [Member] | Corporate Bonds [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Fair Value | 2,749,508 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  | 2,543,402 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |
Level 2 [Member] | Other Mortgage-backed Securities [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Fair Value | 1,194,109 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesOtherMember |  | 817,378 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesOtherMember |
Level 2 [Member] | Equity Securities [Member] | Recurring [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Fair Value | 18,509 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |  | 17,573 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |
Level 2 [Member] | Other investments | Recurring [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Investments, Fair Value | 12,458 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = hmn_OtherPolicyholdersFundsEmbeddedDerivativesMember |  | 5,000 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = hmn_OtherPolicyholdersFundsEmbeddedDerivativesMember |
Level 3 [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Investments, Fair Value | 149,792 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |  | 144,921 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |
Level 3 [Member] | Recurring [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Fair Value | 171,294 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  | 109,529 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
Investments, Fair Value | 171,300 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  | 109,535 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
Separate Account (variable annuity) assets, Fair Value | 0 hmn_SeparateAccountAssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | [1] | 0 hmn_SeparateAccountAssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | [1]
Financial Liabilities Fair value | 20,049 us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  | 0 us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
Level 3 [Member] | Short-term investments [Member] | Recurring [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Investments, Fair Value | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember |  | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember |
Level 3 [Member] | Mortgage-backed securities [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Fair Value | 0 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember |  | 0 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember |
Level 3 [Member] | Other including U.S. Treasury Securities [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Fair Value | 0 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = hmn_OtherIncludingUsTreasurySecuritiesMember |  | 0 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = hmn_OtherIncludingUsTreasurySecuritiesMember |
Level 3 [Member] | Municipal Bonds [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Fair Value | 13,628 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember |  | 2,694 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember |
Level 3 [Member] | Foreign government bonds [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Fair Value | 0 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForeignGovernmentDebtMember |  | 0 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForeignGovernmentDebtMember |
Level 3 [Member] | Corporate Bonds [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Fair Value | 74,717 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  | 60,826 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |
Level 3 [Member] | Other Mortgage-backed Securities [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Fair Value | 82,949 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesOtherMember |  | 46,009 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesOtherMember |
Level 3 [Member] | Equity Securities [Member] | Recurring [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Fair Value | 6 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |  | 6 hmn_AvailableForSaleSecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |
Level 3 [Member] | Other investments | Recurring [Member] |  |  |  |
U.S. government and federally sponsored agency obligations |  |  |  |
Investments, Fair Value | $ 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = hmn_OtherPolicyholdersFundsEmbeddedDerivativesMember |  | $ 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = hmn_OtherPolicyholdersFundsEmbeddedDerivativesMember |
[1] | Separate Account (variable annuity) liabilities are set equal to Separate Account (variable annuity) assets.

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