false FY 0000944695 --12-31 Large Accelerated Filer P3Y P5Y us-gaap:PensionPlansDefinedBenefitMember 0.3333 0.3333 0.3333 0.3333 0.3333 0.3333 600000 600000 600000 us-gaap:MarketApproachValuationTechniqueMember us-gaap:MarketApproachValuationTechniqueMember thg:MeasurementInputNetTangibleAssetMarketMultipleMember thg:MeasurementInputNetTangibleAssetMarketMultipleMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:MeasurementInputDiscountRateMember us-gaap:MeasurementInputDiscountRateMember 0.007 0.068 0.043 0.007 0.068 0.034 0.013 0.013 0.005 0.005 0.003 0.005 0.005 0.025 0.025 0.025 0.025 0.003 0.003 0.003 0.003 0.019 0.031 0.027 0.019 0.031 0.027 0.005 0.005 0.005 0.005 0.003 0.003 0.003 0.003 1.0 1.0 1.0 1.0 0.180 0.180 0.180 0.180 us-gaap:PensionPlansDefinedBenefitMember us-gaap:PensionPlansDefinedBenefitMember us-gaap:PensionPlansDefinedBenefitMember us-gaap:PensionPlansDefinedBenefitMember us-gaap:PensionPlansDefinedBenefitMember us-gaap:PensionPlansDefinedBenefitMember us-gaap:PensionPlansDefinedBenefitMember us-gaap:PensionPlansDefinedBenefitMember us-gaap:PensionPlansDefinedBenefitMember us-gaap:PensionPlansDefinedBenefitMember us-gaap:PensionPlansDefinedBenefitMember 34.66 P2Y9M10D P4Y1M20D 66.14 P5Y2M1D 70.51 P6Y4M2D 82.39 P7Y2M12D 104.11 P8Y1M28D 113.32 P9Y2M1D 40.01 68.41 77.91 85.87 105.53 117.22 0.18495 0.17943 0.18341 0.02527 0.02365 0.01324 0.01740 0.01954 0.02193 0.02286 0.19100 0.18500 0.21694 0.02617 0.02969 0.02201 P2Y6M P6Y6M P2Y6M P6Y P2Y6M P6Y 0000944695 2019-01-01 2019-12-31 iso4217:USD 0000944695 2019-06-28 xbrli:shares 0000944695 2020-02-19 0000944695 us-gaap:CommonStockMember 2019-01-01 2019-12-31 0000944695 thg:SevenPointSixTwoFiveSeniorDebenturesDueTwoThousandTwentyFiveMember 2019-01-01 2019-12-31 0000944695 thg:SixPointThreeFivePercentageSubordinatedDebenturesDueTwoThousandFiftyThreeMember 2019-01-01 2019-12-31 0000944695 2018-01-01 2018-12-31 0000944695 2017-01-01 2017-12-31 0000944695 thg:ChaucerMember 2019-01-01 2019-12-31 0000944695 thg:ChaucerMember 2018-01-01 2018-12-31 0000944695 thg:ChaucerMember 2017-01-01 2017-12-31 0000944695 thg:LifeMember 2019-01-01 2019-12-31 0000944695 thg:LifeMember 2018-01-01 2018-12-31 0000944695 thg:LifeMember 2017-01-01 2017-12-31 iso4217:USD xbrli:shares 0000944695 2019-12-31 0000944695 2018-12-31 0000944695 us-gaap:DiscontinuedOperationsHeldforsaleMember 2018-12-31 0000944695 us-gaap:CommonStockMember 2018-12-31 0000944695 us-gaap:CommonStockMember 2017-12-31 0000944695 us-gaap:CommonStockMember 2016-12-31 0000944695 us-gaap:CommonStockMember 2019-12-31 0000944695 us-gaap:AdditionalPaidInCapitalMember 2018-12-31 0000944695 us-gaap:AdditionalPaidInCapitalMember 2017-12-31 0000944695 us-gaap:AdditionalPaidInCapitalMember 2016-12-31 0000944695 us-gaap:AdditionalPaidInCapitalMember 2019-01-01 2019-12-31 0000944695 us-gaap:AdditionalPaidInCapitalMember 2018-01-01 2018-12-31 0000944695 us-gaap:AdditionalPaidInCapitalMember 2017-01-01 2017-12-31 0000944695 us-gaap:AdditionalPaidInCapitalMember 2019-12-31 0000944695 us-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember 2018-12-31 0000944695 us-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember 2017-12-31 0000944695 us-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember 2016-12-31 0000944695 us-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember 2019-01-01 2019-12-31 0000944695 us-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember 2018-01-01 2018-12-31 0000944695 us-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember 2017-01-01 2017-12-31 0000944695 us-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember thg:AcceleratedShareRepurchaseProgramMember 2019-01-01 2019-12-31 0000944695 us-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember thg:AcceleratedShareRepurchaseProgramMember 2018-01-01 2018-12-31 0000944695 us-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember 2019-12-31 0000944695 us-gaap:AccumulatedDefinedBenefitPlansAdjustmentMember 2018-12-31 0000944695 us-gaap:AccumulatedDefinedBenefitPlansAdjustmentMember 2017-12-31 0000944695 us-gaap:AccumulatedDefinedBenefitPlansAdjustmentMember 2016-12-31 0000944695 us-gaap:AccumulatedDefinedBenefitPlansAdjustmentMember 2019-01-01 2019-12-31 0000944695 us-gaap:AccumulatedDefinedBenefitPlansAdjustmentMember 2018-01-01 2018-12-31 0000944695 us-gaap:AccumulatedDefinedBenefitPlansAdjustmentMember 2017-01-01 2017-12-31 0000944695 us-gaap:AccumulatedDefinedBenefitPlansAdjustmentMember us-gaap:AccountingStandardsUpdate201802Member 2018-01-01 2018-12-31 0000944695 us-gaap:AccumulatedDefinedBenefitPlansAdjustmentMember 2019-12-31 0000944695 us-gaap:AccumulatedTranslationAdjustmentMember 2018-12-31 0000944695 us-gaap:AccumulatedTranslationAdjustmentMember 2017-12-31 0000944695 us-gaap:AccumulatedTranslationAdjustmentMember 2016-12-31 0000944695 us-gaap:AccumulatedTranslationAdjustmentMember 2018-01-01 2018-12-31 0000944695 us-gaap:AccumulatedTranslationAdjustmentMember 2017-01-01 2017-12-31 0000944695 us-gaap:AccumulatedTranslationAdjustmentMember us-gaap:AccountingStandardsUpdate201802Member 2018-01-01 2018-12-31 0000944695 us-gaap:AccumulatedTranslationAdjustmentMember 2019-01-01 2019-12-31 0000944695 us-gaap:AccumulatedOtherComprehensiveIncomeMember 2019-12-31 0000944695 us-gaap:AccumulatedOtherComprehensiveIncomeMember 2018-12-31 0000944695 us-gaap:AccumulatedOtherComprehensiveIncomeMember 2017-12-31 0000944695 us-gaap:RetainedEarningsMember 2018-12-31 0000944695 us-gaap:RetainedEarningsMember 2017-12-31 0000944695 us-gaap:RetainedEarningsMember 2016-12-31 0000944695 us-gaap:RetainedEarningsMember 2019-01-01 2019-12-31 0000944695 us-gaap:RetainedEarningsMember 2018-01-01 2018-12-31 0000944695 us-gaap:RetainedEarningsMember 2017-01-01 2017-12-31 0000944695 us-gaap:RetainedEarningsMember 2019-12-31 0000944695 us-gaap:TreasuryStockMember 2018-12-31 0000944695 us-gaap:TreasuryStockMember 2017-12-31 0000944695 us-gaap:TreasuryStockMember 2016-12-31 0000944695 us-gaap:TreasuryStockMember 2019-01-01 2019-12-31 0000944695 us-gaap:TreasuryStockMember 2018-01-01 2018-12-31 0000944695 us-gaap:TreasuryStockMember 2017-01-01 2017-12-31 0000944695 us-gaap:TreasuryStockMember 2019-12-31 0000944695 2017-12-31 0000944695 2016-12-31 0000944695 us-gaap:FairValueMeasuredAtNetAssetValuePerShareMember 2019-01-01 2019-12-31 0000944695 srt:MinimumMember 2019-01-01 2019-12-31 0000944695 srt:MaximumMember 2019-01-01 2019-12-31 0000944695 srt:MinimumMember us-gaap:SoftwareAndSoftwareDevelopmentCostsMember 2019-01-01 2019-12-31 0000944695 srt:MaximumMember us-gaap:SoftwareAndSoftwareDevelopmentCostsMember 2019-01-01 2019-12-31 0000944695 srt:MinimumMember thg:RealEstateAndFleetLeasesMember 2019-01-01 2019-12-31 0000944695 thg:RealEstateAndFleetLeasesMember srt:MaximumMember 2019-01-01 2019-12-31 0000944695 2019-10-01 2019-12-31 0000944695 2018-10-01 2018-12-31 xbrli:pure 0000944695 us-gaap:AccountingStandardsUpdate201802Member 2018-01-01 2018-01-01 0000944695 us-gaap:AccountingStandardsUpdate201802Member 2019-01-01 2019-01-01 0000944695 thg:AccountingStandardsUpdate201602And201811Member 2019-01-01 0000944695 thg:AccountingStandardsUpdate201601And201803Member 2019-12-31 0000944695 thg:AccountingStandardsUpdate201601And201803Member 2019-01-01 2019-12-31 0000944695 thg:ChaucerMember 2018-12-28 2018-12-28 0000944695 thg:ChaucerMember 2018-12-31 0000944695 thg:ChaucerIrishEntityMember 2018-01-01 2018-12-31 0000944695 thg:ChaucerIrishEntityMember 2019-02-13 2019-02-14 0000944695 thg:AustralianEntitiesMember 2018-01-01 2018-03-31 0000944695 thg:ChaucerMember 2018-04-01 2018-06-30 0000944695 thg:ChaucerMember thg:ChinaReinsuranceGroupCorporationMember 2018-01-01 2018-12-31 0000944695 srt:MaximumMember 2018-01-01 2018-12-31 0000944695 thg:ChaucerMember us-gaap:DomesticCountryMember 2019-01-01 2019-12-31 0000944695 thg:ChaucerMember 2019-07-01 2019-09-30 0000944695 thg:AustralianEntitiesMember 2019-04-10 2019-04-10 0000944695 us-gaap:DiscontinuedOperationsHeldforsaleMember 2019-12-31 0000944695 us-gaap:AccidentAndHealthInsuranceSegmentMember 2019-12-31 0000944695 us-gaap:AccidentAndHealthInsuranceSegmentMember 2018-12-31 0000944695 us-gaap:AccidentAndHealthInsuranceSegmentMember 2019-01-01 2019-12-31 0000944695 us-gaap:AccidentAndHealthInsuranceSegmentMember 2018-01-01 2018-12-31 0000944695 us-gaap:AccidentAndHealthInsuranceSegmentMember 2017-01-01 2017-12-31 0000944695 us-gaap:USTreasuryAndGovernmentMember 2019-12-31 0000944695 us-gaap:ForeignGovernmentDebtSecuritiesMember 2019-12-31 0000944695 us-gaap:MunicipalBondsMember 2019-12-31 0000944695 us-gaap:CorporateDebtSecuritiesMember 2019-12-31 0000944695 us-gaap:ResidentialMortgageBackedSecuritiesMember 2019-12-31 0000944695 us-gaap:CommercialMortgageBackedSecuritiesMember 2019-12-31 0000944695 us-gaap:AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember 2019-12-31 0000944695 thg:FixedMaturitiesIncludingHeldForSaleMember 2019-12-31 0000944695 us-gaap:USTreasuryAndGovernmentMember 2018-12-31 0000944695 us-gaap:ForeignGovernmentDebtSecuritiesMember 2018-12-31 0000944695 us-gaap:MunicipalBondsMember 2018-12-31 0000944695 us-gaap:CorporateDebtSecuritiesMember 2018-12-31 0000944695 us-gaap:ResidentialMortgageBackedSecuritiesMember 2018-12-31 0000944695 us-gaap:CommercialMortgageBackedSecuritiesMember 2018-12-31 0000944695 us-gaap:AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember 2018-12-31 0000944695 thg:FixedMaturitiesExcludingHeldForSaleMember 2018-12-31 0000944695 thg:FixedMaturitiesHeldForSaleMember 2018-12-31 0000944695 thg:FixedMaturitiesIncludingHeldForSaleMember 2018-12-31 0000944695 thg:SecurityLendingProgramMember 2018-12-31 0000944695 us-gaap:FederalHomeLoanBankBorrowingsMember 2019-12-31 0000944695 us-gaap:FederalHomeLoanBankBorrowingsMember 2018-12-31 0000944695 us-gaap:DebtSecuritiesMember us-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember 2018-12-31 0000944695 us-gaap:DebtSecuritiesMember us-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember 2019-01-01 2019-12-31 0000944695 us-gaap:DebtSecuritiesMember us-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember 2019-12-31 0000944695 us-gaap:DebtSecuritiesMember us-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember 2017-12-31 0000944695 thg:EquitySecuritiesAndOtherMember us-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember 2017-12-31 0000944695 us-gaap:DebtSecuritiesMember us-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember 2018-01-01 2018-12-31 0000944695 thg:EquitySecuritiesAndOtherMember us-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember 2018-01-01 2018-12-31 0000944695 us-gaap:DebtSecuritiesMember us-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember 2016-12-31 0000944695 thg:EquitySecuritiesAndOtherMember us-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember 2016-12-31 0000944695 us-gaap:DebtSecuritiesMember us-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember 2017-01-01 2017-12-31 0000944695 thg:EquitySecuritiesAndOtherMember us-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember 2017-01-01 2017-12-31 0000944695 us-gaap:USTreasuryAndGovernmentMember thg:InvestmentGradeMember 2019-12-31 0000944695 us-gaap:MunicipalBondsMember thg:InvestmentGradeMember 2019-12-31 0000944695 us-gaap:CorporateDebtSecuritiesMember thg:InvestmentGradeMember 2019-12-31 0000944695 us-gaap:ResidentialMortgageBackedSecuritiesMember thg:InvestmentGradeMember 2019-12-31 0000944695 us-gaap:CommercialMortgageBackedSecuritiesMember thg:InvestmentGradeMember 2019-12-31 0000944695 thg:InvestmentGradeMember 2019-12-31 0000944695 us-gaap:CorporateDebtSecuritiesMember thg:BelowInvestmentGradeMember 2019-12-31 0000944695 thg:FixedMaturitiesIncludingHeldForSaleMember 2019-12-31 0000944695 us-gaap:AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember thg:InvestmentGradeMember 2019-12-31 0000944695 us-gaap:USTreasuryAndGovernmentMember thg:InvestmentGradeMember 2018-12-31 0000944695 us-gaap:MunicipalBondsMember thg:InvestmentGradeMember 2018-12-31 0000944695 us-gaap:CorporateDebtSecuritiesMember thg:InvestmentGradeMember 2018-12-31 0000944695 us-gaap:ResidentialMortgageBackedSecuritiesMember thg:InvestmentGradeMember 2018-12-31 0000944695 us-gaap:CommercialMortgageBackedSecuritiesMember thg:InvestmentGradeMember 2018-12-31 0000944695 us-gaap:AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember thg:InvestmentGradeMember 2018-12-31 0000944695 thg:InvestmentGradeMember 2018-12-31 0000944695 us-gaap:CorporateDebtSecuritiesMember thg:BelowInvestmentGradeMember 2018-12-31 0000944695 thg:BelowInvestmentGradeMember 2018-12-31 0000944695 thg:FixedMaturitiesExcludingHeldForSaleMember 2018-12-31 0000944695 thg:FixedMaturitiesHeldForSaleMember 2018-12-31 0000944695 thg:FixedMaturitiesIncludingHeldForSaleMember 2018-12-31 0000944695 us-gaap:ForeignGovernmentDebtSecuritiesMember thg:InvestmentGradeMember 2018-12-31 0000944695 us-gaap:MunicipalBondsMember thg:BelowInvestmentGradeMember 2018-12-31 0000944695 us-gaap:MortgagesMember 2019-12-31 0000944695 us-gaap:MortgagesMember 2018-12-31 0000944695 us-gaap:MortgagesMember thg:OfficeMember 2019-12-31 0000944695 us-gaap:MortgagesMember thg:OfficeMember 2018-12-31 0000944695 us-gaap:MortgagesMember thg:ApartmentsMember 2019-12-31 0000944695 us-gaap:MortgagesMember thg:ApartmentsMember 2018-12-31 0000944695 us-gaap:MortgagesMember thg:RetailPropertyMember 2019-12-31 0000944695 us-gaap:MortgagesMember thg:RetailPropertyMember 2018-12-31 0000944695 us-gaap:MortgagesMember thg:HotelPropertyMember 2019-12-31 0000944695 us-gaap:MortgagesMember thg:HotelPropertyMember 2018-12-31 0000944695 us-gaap:MortgagesMember thg:IndustrialMember 2019-12-31 0000944695 us-gaap:MortgagesMember thg:IndustrialMember 2018-12-31 0000944695 us-gaap:MortgagesMember thg:PacificMember 2019-12-31 0000944695 us-gaap:MortgagesMember thg:PacificMember 2018-12-31 0000944695 us-gaap:MortgagesMember thg:SouthAtlanticMember 2019-12-31 0000944695 us-gaap:MortgagesMember thg:SouthAtlanticMember 2018-12-31 0000944695 us-gaap:MortgagesMember thg:WestSouthCentralMember 2019-12-31 0000944695 us-gaap:MortgagesMember thg:WestSouthCentralMember 2018-12-31 0000944695 us-gaap:MortgagesMember thg:MidAtlanticMember 2019-12-31 0000944695 us-gaap:MortgagesMember thg:MidAtlanticMember 2018-12-31 0000944695 us-gaap:MortgagesMember thg:NewEnglandMember 2019-12-31 0000944695 us-gaap:MortgagesMember thg:NewEnglandMember 2018-12-31 0000944695 us-gaap:MortgagesMember thg:EastNorthCentralMember 2019-12-31 0000944695 us-gaap:MortgagesMember thg:EastNorthCentralMember 2018-12-31 0000944695 us-gaap:MortgagesMember thg:MountainMember 2019-12-31 0000944695 us-gaap:MortgagesMember thg:MountainMember 2018-12-31 0000944695 us-gaap:MortgagesMember thg:OtherLocationsMember 2019-12-31 0000944695 us-gaap:MortgagesMember thg:OtherLocationsMember 2018-12-31 0000944695 us-gaap:MortgagesMember 2018-01-01 2018-12-31 0000944695 srt:PartnershipInterestMember 2019-12-31 0000944695 srt:PartnershipInterestMember 2018-12-31 0000944695 us-gaap:USGovernmentSponsoredEnterprisesDebtSecuritiesMember 2019-12-31 0000944695 us-gaap:USGovernmentSponsoredEnterprisesDebtSecuritiesMember 2018-12-31 0000944695 thg:MortgageParticipationsMember 2019-12-31 0000944695 thg:MortgageParticipationsMember 2018-12-31 0000944695 us-gaap:DebtSecuritiesMember 2019-01-01 2019-12-31 0000944695 us-gaap:DebtSecuritiesMember 2018-01-01 2018-12-31 0000944695 us-gaap:DebtSecuritiesMember 2017-01-01 2017-12-31 0000944695 srt:PartnershipInterestMember 2019-01-01 2019-12-31 0000944695 srt:PartnershipInterestMember 2018-01-01 2018-12-31 0000944695 srt:PartnershipInterestMember 2017-01-01 2017-12-31 0000944695 us-gaap:EquitySecuritiesMember 2019-01-01 2019-12-31 0000944695 us-gaap:EquitySecuritiesMember 2018-01-01 2018-12-31 0000944695 us-gaap:EquitySecuritiesMember 2017-01-01 2017-12-31 0000944695 us-gaap:MortgagesMember 2019-01-01 2019-12-31 0000944695 us-gaap:MortgagesMember 2017-01-01 2017-12-31 0000944695 us-gaap:OtherThanSecuritiesInvestmentMember 2019-01-01 2019-12-31 0000944695 us-gaap:OtherThanSecuritiesInvestmentMember 2018-01-01 2018-12-31 0000944695 us-gaap:OtherThanSecuritiesInvestmentMember 2017-01-01 2017-12-31 0000944695 us-gaap:AccountingStandardsUpdate201601Member 2019-01-01 2019-12-31 0000944695 us-gaap:AccountingStandardsUpdate201601Member 2018-01-01 2018-12-31 0000944695 us-gaap:AccumulatedOtherComprehensiveIncomeMember 2017-01-01 2017-12-31 0000944695 thg:EquitySecuritiesAndOtherMember 2017-01-01 2017-12-31 0000944695 thg:FixedMaturitiesExcludingHeldForSaleMember 2019-01-01 2019-12-31 0000944695 thg:FixedMaturitiesHeldForSaleMember 2019-01-01 2019-12-31 0000944695 thg:FixedMaturitiesIncludingHeldForSaleMember 2019-01-01 2019-12-31 0000944695 thg:FixedMaturitiesExcludingHeldForSaleMember 2018-01-01 2018-12-31 0000944695 thg:FixedMaturitiesHeldForSaleMember 2018-01-01 2018-12-31 0000944695 thg:FixedMaturitiesIncludingHeldForSaleMember 2018-01-01 2018-12-31 0000944695 thg:FixedMaturitiesExcludingHeldForSaleMember 2017-01-01 2017-12-31 0000944695 thg:FixedMaturitiesHeldForSaleMember 2017-01-01 2017-12-31 0000944695 thg:FixedMaturitiesIncludingHeldForSaleMember 2017-01-01 2017-12-31 0000944695 us-gaap:CarryingReportedAmountFairValueDisclosureMember us-gaap:ValuationTechniqueDiscountedCashFlowMember thg:FairValueThroughAccumulatedOtherComprehensiveIncomeMember us-gaap:DebtSecuritiesMember 2019-12-31 0000944695 us-gaap:EstimateOfFairValueFairValueDisclosureMember us-gaap:ValuationTechniqueDiscountedCashFlowMember thg:FairValueThroughAccumulatedOtherComprehensiveIncomeMember us-gaap:DebtSecuritiesMember 2019-12-31 0000944695 us-gaap:CarryingReportedAmountFairValueDisclosureMember us-gaap:ValuationTechniqueDiscountedCashFlowMember thg:FairValueThroughAccumulatedOtherComprehensiveIncomeMember us-gaap:DebtSecuritiesMember 2018-12-31 0000944695 us-gaap:EstimateOfFairValueFairValueDisclosureMember us-gaap:ValuationTechniqueDiscountedCashFlowMember thg:FairValueThroughAccumulatedOtherComprehensiveIncomeMember us-gaap:DebtSecuritiesMember 2018-12-31 0000944695 us-gaap:CarryingReportedAmountFairValueDisclosureMember us-gaap:ValuationTechniqueDiscountedCashFlowMember thg:FairValueThroughNetIncomeMember us-gaap:EquitySecuritiesMember 2019-12-31 0000944695 us-gaap:EstimateOfFairValueFairValueDisclosureMember us-gaap:ValuationTechniqueDiscountedCashFlowMember thg:FairValueThroughNetIncomeMember us-gaap:EquitySecuritiesMember 2019-12-31 0000944695 us-gaap:CarryingReportedAmountFairValueDisclosureMember us-gaap:ValuationTechniqueDiscountedCashFlowMember thg:FairValueThroughNetIncomeMember us-gaap:EquitySecuritiesMember 2018-12-31 0000944695 us-gaap:EstimateOfFairValueFairValueDisclosureMember us-gaap:ValuationTechniqueDiscountedCashFlowMember thg:FairValueThroughNetIncomeMember us-gaap:EquitySecuritiesMember 2018-12-31 0000944695 us-gaap:CarryingReportedAmountFairValueDisclosureMember us-gaap:ValuationTechniqueDiscountedCashFlowMember thg:FairValueThroughNetIncomeMember us-gaap:OtherInvestmentsMember 2019-12-31 0000944695 us-gaap:EstimateOfFairValueFairValueDisclosureMember us-gaap:ValuationTechniqueDiscountedCashFlowMember thg:FairValueThroughNetIncomeMember us-gaap:OtherInvestmentsMember 2019-12-31 0000944695 us-gaap:CarryingReportedAmountFairValueDisclosureMember us-gaap:ValuationTechniqueDiscountedCashFlowMember thg:FairValueThroughNetIncomeMember us-gaap:OtherInvestmentsMember 2018-12-31 0000944695 us-gaap:EstimateOfFairValueFairValueDisclosureMember us-gaap:ValuationTechniqueDiscountedCashFlowMember thg:FairValueThroughNetIncomeMember us-gaap:OtherInvestmentsMember 2018-12-31 0000944695 us-gaap:CarryingReportedAmountFairValueDisclosureMember us-gaap:CostApproachValuationTechniqueMember thg:AmortizedCostOrCostMember us-gaap:OtherInvestmentsMember 2019-12-31 0000944695 us-gaap:EstimateOfFairValueFairValueDisclosureMember us-gaap:CostApproachValuationTechniqueMember thg:AmortizedCostOrCostMember us-gaap:OtherInvestmentsMember 2019-12-31 0000944695 us-gaap:CarryingReportedAmountFairValueDisclosureMember us-gaap:CostApproachValuationTechniqueMember thg:AmortizedCostOrCostMember us-gaap:OtherInvestmentsMember 2018-12-31 0000944695 us-gaap:EstimateOfFairValueFairValueDisclosureMember us-gaap:CostApproachValuationTechniqueMember thg:AmortizedCostOrCostMember us-gaap:OtherInvestmentsMember 2018-12-31 0000944695 us-gaap:CarryingReportedAmountFairValueDisclosureMember us-gaap:CostApproachValuationTechniqueMember thg:AmortizedCostOrCostMember 2019-12-31 0000944695 us-gaap:EstimateOfFairValueFairValueDisclosureMember us-gaap:CostApproachValuationTechniqueMember thg:AmortizedCostOrCostMember 2019-12-31 0000944695 us-gaap:CarryingReportedAmountFairValueDisclosureMember us-gaap:CostApproachValuationTechniqueMember thg:AmortizedCostOrCostMember 2018-12-31 0000944695 us-gaap:EstimateOfFairValueFairValueDisclosureMember us-gaap:CostApproachValuationTechniqueMember thg:AmortizedCostOrCostMember 2018-12-31 0000944695 us-gaap:CarryingReportedAmountFairValueDisclosureMember 2019-12-31 0000944695 us-gaap:EstimateOfFairValueFairValueDisclosureMember 2019-12-31 0000944695 us-gaap:CarryingReportedAmountFairValueDisclosureMember 2018-12-31 0000944695 us-gaap:EstimateOfFairValueFairValueDisclosureMember 2018-12-31 0000944695 us-gaap:CarryingReportedAmountFairValueDisclosureMember thg:ChaucerMember 2018-12-31 0000944695 us-gaap:EstimateOfFairValueFairValueDisclosureMember thg:ChaucerMember 2018-12-31 0000944695 thg:USTreasurySecuritiesAndUSGovernmentAndAgencySecuritiesMember us-gaap:FairValueMeasurementsRecurringMember 2019-12-31 0000944695 thg:USTreasurySecuritiesAndUSGovernmentAndAgencySecuritiesMember us-gaap:FairValueInputsLevel1Member us-gaap:FairValueMeasurementsRecurringMember 2019-12-31 0000944695 thg:USTreasurySecuritiesAndUSGovernmentAndAgencySecuritiesMember us-gaap:FairValueInputsLevel2Member us-gaap:FairValueMeasurementsRecurringMember 2019-12-31 0000944695 us-gaap:ForeignGovernmentDebtSecuritiesMember us-gaap:FairValueMeasurementsRecurringMember 2019-12-31 0000944695 us-gaap:ForeignGovernmentDebtSecuritiesMember us-gaap:FairValueInputsLevel2Member us-gaap:FairValueMeasurementsRecurringMember 2019-12-31 0000944695 us-gaap:MunicipalBondsMember us-gaap:FairValueMeasurementsRecurringMember 2019-12-31 0000944695 us-gaap:MunicipalBondsMember us-gaap:FairValueInputsLevel2Member us-gaap:FairValueMeasurementsRecurringMember 2019-12-31 0000944695 us-gaap:MunicipalBondsMember us-gaap:FairValueInputsLevel3Member us-gaap:FairValueMeasurementsRecurringMember 2019-12-31 0000944695 us-gaap:CorporateDebtSecuritiesMember us-gaap:FairValueMeasurementsRecurringMember 2019-12-31 0000944695 us-gaap:CorporateDebtSecuritiesMember us-gaap:FairValueInputsLevel2Member us-gaap:FairValueMeasurementsRecurringMember 2019-12-31 0000944695 us-gaap:CorporateDebtSecuritiesMember us-gaap:FairValueInputsLevel3Member us-gaap:FairValueMeasurementsRecurringMember 2019-12-31 0000944695 thg:ResidentialMortgageBackedSecuritiesUSAgencyBackedMember us-gaap:FairValueMeasurementsRecurringMember 2019-12-31 0000944695 thg:ResidentialMortgageBackedSecuritiesUSAgencyBackedMember us-gaap:FairValueInputsLevel2Member us-gaap:FairValueMeasurementsRecurringMember 2019-12-31 0000944695 us-gaap:ResidentialMortgageBackedSecuritiesMember us-gaap:FairValueMeasurementsRecurringMember 2019-12-31 0000944695 us-gaap:ResidentialMortgageBackedSecuritiesMember us-gaap:FairValueInputsLevel2Member us-gaap:FairValueMeasurementsRecurringMember 2019-12-31 0000944695 us-gaap:CommercialMortgageBackedSecuritiesMember us-gaap:FairValueMeasurementsRecurringMember 2019-12-31 0000944695 us-gaap:CommercialMortgageBackedSecuritiesMember us-gaap:FairValueInputsLevel2Member us-gaap:FairValueMeasurementsRecurringMember 2019-12-31 0000944695 us-gaap:CommercialMortgageBackedSecuritiesMember us-gaap:FairValueInputsLevel3Member us-gaap:FairValueMeasurementsRecurringMember 2019-12-31 0000944695 us-gaap:AssetBackedSecuritiesMember us-gaap:FairValueMeasurementsRecurringMember 2019-12-31 0000944695 us-gaap:AssetBackedSecuritiesMember us-gaap:FairValueInputsLevel2Member us-gaap:FairValueMeasurementsRecurringMember 2019-12-31 0000944695 us-gaap:FairValueMeasurementsRecurringMember 2019-12-31 0000944695 us-gaap:FairValueInputsLevel1Member us-gaap:FairValueMeasurementsRecurringMember 2019-12-31 0000944695 us-gaap:FairValueInputsLevel2Member us-gaap:FairValueMeasurementsRecurringMember 2019-12-31 0000944695 us-gaap:FairValueInputsLevel3Member us-gaap:FairValueMeasurementsRecurringMember 2019-12-31 0000944695 thg:USTreasurySecuritiesAndUSGovernmentAndAgencySecuritiesMember us-gaap:FairValueMeasurementsRecurringMember 2018-12-31 0000944695 thg:USTreasurySecuritiesAndUSGovernmentAndAgencySecuritiesMember us-gaap:FairValueInputsLevel1Member us-gaap:FairValueMeasurementsRecurringMember 2018-12-31 0000944695 thg:USTreasurySecuritiesAndUSGovernmentAndAgencySecuritiesMember us-gaap:FairValueInputsLevel2Member us-gaap:FairValueMeasurementsRecurringMember 2018-12-31 0000944695 us-gaap:ForeignGovernmentDebtSecuritiesMember us-gaap:FairValueMeasurementsRecurringMember 2018-12-31 0000944695 us-gaap:ForeignGovernmentDebtSecuritiesMember us-gaap:FairValueInputsLevel2Member us-gaap:FairValueMeasurementsRecurringMember 2018-12-31 0000944695 us-gaap:MunicipalBondsMember us-gaap:FairValueMeasurementsRecurringMember 2018-12-31 0000944695 us-gaap:MunicipalBondsMember us-gaap:FairValueInputsLevel2Member us-gaap:FairValueMeasurementsRecurringMember 2018-12-31 0000944695 us-gaap:MunicipalBondsMember us-gaap:FairValueInputsLevel3Member us-gaap:FairValueMeasurementsRecurringMember 2018-12-31 0000944695 us-gaap:CorporateDebtSecuritiesMember us-gaap:FairValueMeasurementsRecurringMember 2018-12-31 0000944695 us-gaap:CorporateDebtSecuritiesMember us-gaap:FairValueInputsLevel2Member us-gaap:FairValueMeasurementsRecurringMember 2018-12-31 0000944695 us-gaap:CorporateDebtSecuritiesMember us-gaap:FairValueInputsLevel3Member us-gaap:FairValueMeasurementsRecurringMember 2018-12-31 0000944695 thg:ResidentialMortgageBackedSecuritiesUSAgencyBackedMember us-gaap:FairValueMeasurementsRecurringMember 2018-12-31 0000944695 thg:ResidentialMortgageBackedSecuritiesUSAgencyBackedMember us-gaap:FairValueInputsLevel2Member us-gaap:FairValueMeasurementsRecurringMember 2018-12-31 0000944695 us-gaap:ResidentialMortgageBackedSecuritiesMember us-gaap:FairValueMeasurementsRecurringMember 2018-12-31 0000944695 us-gaap:ResidentialMortgageBackedSecuritiesMember us-gaap:FairValueInputsLevel2Member us-gaap:FairValueMeasurementsRecurringMember 2018-12-31 0000944695 us-gaap:CommercialMortgageBackedSecuritiesMember us-gaap:FairValueMeasurementsRecurringMember 2018-12-31 0000944695 us-gaap:CommercialMortgageBackedSecuritiesMember us-gaap:FairValueInputsLevel2Member us-gaap:FairValueMeasurementsRecurringMember 2018-12-31 0000944695 us-gaap:CommercialMortgageBackedSecuritiesMember us-gaap:FairValueInputsLevel3Member us-gaap:FairValueMeasurementsRecurringMember 2018-12-31 0000944695 us-gaap:AssetBackedSecuritiesMember us-gaap:FairValueMeasurementsRecurringMember 2018-12-31 0000944695 us-gaap:AssetBackedSecuritiesMember us-gaap:FairValueInputsLevel2Member us-gaap:FairValueMeasurementsRecurringMember 2018-12-31 0000944695 us-gaap:FairValueMeasurementsRecurringMember 2018-12-31 0000944695 us-gaap:FairValueInputsLevel1Member us-gaap:FairValueMeasurementsRecurringMember 2018-12-31 0000944695 us-gaap:FairValueInputsLevel2Member us-gaap:FairValueMeasurementsRecurringMember 2018-12-31 0000944695 us-gaap:FairValueInputsLevel3Member us-gaap:FairValueMeasurementsRecurringMember 2018-12-31 0000944695 us-gaap:FairValueMeasurementsRecurringMember thg:ChaucerMember 2018-12-31 0000944695 us-gaap:FairValueInputsLevel1Member us-gaap:FairValueMeasurementsRecurringMember thg:ChaucerMember 2018-12-31 0000944695 us-gaap:FairValueInputsLevel2Member us-gaap:FairValueMeasurementsRecurringMember thg:ChaucerMember 2018-12-31 0000944695 us-gaap:FairValueMeasuredAtNetAssetValuePerShareMember srt:PartnershipInterestMember 2019-12-31 0000944695 us-gaap:FairValueMeasuredAtNetAssetValuePerShareMember srt:PartnershipInterestMember 2018-12-31 0000944695 us-gaap:FairValueInputsLevel1Member us-gaap:EstimateOfFairValueFairValueDisclosureMember 2019-12-31 0000944695 us-gaap:FairValueInputsLevel2Member us-gaap:EstimateOfFairValueFairValueDisclosureMember 2019-12-31 0000944695 us-gaap:FairValueInputsLevel3Member us-gaap:EstimateOfFairValueFairValueDisclosureMember 2019-12-31 0000944695 us-gaap:EstimateOfFairValueFairValueDisclosureMember us-gaap:FairValueInputsLevel1Member 2018-12-31 0000944695 us-gaap:EstimateOfFairValueFairValueDisclosureMember us-gaap:FairValueInputsLevel2Member 2018-12-31 0000944695 us-gaap:EstimateOfFairValueFairValueDisclosureMember us-gaap:FairValueInputsLevel3Member 2018-12-31 0000944695 us-gaap:EstimateOfFairValueFairValueDisclosureMember us-gaap:FairValueInputsLevel1Member thg:ChaucerMember 2018-12-31 0000944695 us-gaap:CorporateDebtSecuritiesMember 2018-12-31 0000944695 us-gaap:CommercialMortgageBackedSecuritiesMember 2018-12-31 0000944695 us-gaap:DebtSecuritiesMember 2018-12-31 0000944695 thg:EquitySecuritiesAndOtherMember 2018-12-31 0000944695 us-gaap:MunicipalBondsMember 2019-01-01 2019-12-31 0000944695 us-gaap:CommercialMortgageBackedSecuritiesMember 2019-01-01 2019-12-31 0000944695 thg:EquitySecuritiesAndOtherMember 2019-01-01 2019-12-31 0000944695 us-gaap:CorporateDebtSecuritiesMember 2019-01-01 2019-12-31 0000944695 us-gaap:CorporateDebtSecuritiesMember 2019-12-31 0000944695 us-gaap:CommercialMortgageBackedSecuritiesMember 2019-12-31 0000944695 us-gaap:DebtSecuritiesMember 2019-12-31 0000944695 thg:EquitySecuritiesAndOtherMember 2019-12-31 0000944695 us-gaap:MunicipalBondsMember 2017-12-31 0000944695 us-gaap:CorporateDebtSecuritiesMember 2017-12-31 0000944695 us-gaap:CommercialMortgageBackedSecuritiesMember 2017-12-31 0000944695 us-gaap:DebtSecuritiesMember 2017-12-31 0000944695 thg:EquitySecuritiesAndOtherMember 2017-12-31 0000944695 us-gaap:MunicipalBondsMember 2018-01-01 2018-12-31 0000944695 thg:EquitySecuritiesAndOtherMember 2018-01-01 2018-12-31 0000944695 us-gaap:CommercialMortgageBackedSecuritiesMember 2018-01-01 2018-12-31 0000944695 us-gaap:CorporateDebtSecuritiesMember 2018-01-01 2018-12-31 0000944695 us-gaap:FairValueInputsLevel3Member 2019-12-31 0000944695 us-gaap:FairValueInputsLevel3Member 2018-12-31 0000944695 us-gaap:MunicipalBondsMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:FairValueInputsLevel3Member 2019-12-31 0000944695 us-gaap:MunicipalBondsMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:FairValueInputsLevel3Member 2018-12-31 0000944695 us-gaap:CorporateDebtSecuritiesMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:FairValueInputsLevel3Member 2019-12-31 0000944695 us-gaap:CorporateDebtSecuritiesMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:FairValueInputsLevel3Member 2018-12-31 0000944695 us-gaap:CommercialMortgageBackedSecuritiesMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:FairValueInputsLevel3Member 2019-12-31 0000944695 us-gaap:CommercialMortgageBackedSecuritiesMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:FairValueInputsLevel3Member 2018-12-31 0000944695 srt:MinimumMember us-gaap:MunicipalBondsMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:FairValueInputsLevel3Member thg:MeasurementInputDiscountForSmallIssueSizeMember 2019-01-01 2019-12-31 0000944695 srt:MaximumMember us-gaap:MunicipalBondsMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:FairValueInputsLevel3Member thg:MeasurementInputDiscountForSmallIssueSizeMember 2019-01-01 2019-12-31 0000944695 srt:WeightedAverageMember us-gaap:MunicipalBondsMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:FairValueInputsLevel3Member thg:MeasurementInputDiscountForSmallIssueSizeMember 2019-01-01 2019-12-31 0000944695 srt:MinimumMember us-gaap:MunicipalBondsMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:FairValueInputsLevel3Member thg:MeasurementInputDiscountForSmallIssueSizeMember 2018-01-01 2018-12-31 0000944695 srt:MaximumMember us-gaap:MunicipalBondsMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:FairValueInputsLevel3Member thg:MeasurementInputDiscountForSmallIssueSizeMember 2018-01-01 2018-12-31 0000944695 srt:WeightedAverageMember us-gaap:MunicipalBondsMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:FairValueInputsLevel3Member thg:MeasurementInputDiscountForSmallIssueSizeMember 2018-01-01 2018-12-31 0000944695 us-gaap:MunicipalBondsMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:FairValueInputsLevel3Member thg:MeasurementInputDiscountForCreditStressMember 2018-01-01 2018-12-31 0000944695 srt:WeightedAverageMember us-gaap:MunicipalBondsMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:FairValueInputsLevel3Member thg:MeasurementInputDiscountForCreditStressMember 2018-01-01 2018-12-31 0000944695 us-gaap:MunicipalBondsMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:FairValueInputsLevel3Member thg:MeasurementInputDiscountRateForAboveMarketCouponMember 2019-01-01 2019-12-31 0000944695 srt:WeightedAverageMember us-gaap:MunicipalBondsMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:FairValueInputsLevel3Member thg:MeasurementInputDiscountRateForAboveMarketCouponMember 2019-01-01 2019-12-31 0000944695 srt:MinimumMember us-gaap:MunicipalBondsMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:FairValueInputsLevel3Member thg:MeasurementInputDiscountRateForAboveMarketCouponMember 2018-01-01 2018-12-31 0000944695 srt:MaximumMember us-gaap:MunicipalBondsMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:FairValueInputsLevel3Member thg:MeasurementInputDiscountRateForAboveMarketCouponMember 2018-01-01 2018-12-31 0000944695 srt:WeightedAverageMember us-gaap:MunicipalBondsMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:FairValueInputsLevel3Member thg:MeasurementInputDiscountRateForAboveMarketCouponMember 2018-01-01 2018-12-31 0000944695 us-gaap:CorporateDebtSecuritiesMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:FairValueInputsLevel3Member us-gaap:MeasurementInputDiscountRateMember 2019-12-31 0000944695 srt:WeightedAverageMember us-gaap:CorporateDebtSecuritiesMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:FairValueInputsLevel3Member us-gaap:MeasurementInputDiscountRateMember 2019-12-31 0000944695 us-gaap:CorporateDebtSecuritiesMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:FairValueInputsLevel3Member us-gaap:MeasurementInputDiscountRateMember 2018-12-31 0000944695 srt:WeightedAverageMember us-gaap:CorporateDebtSecuritiesMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:FairValueInputsLevel3Member us-gaap:MeasurementInputDiscountRateMember 2018-12-31 0000944695 srt:MinimumMember us-gaap:CommercialMortgageBackedSecuritiesMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:FairValueInputsLevel3Member thg:MeasurementInputDiscountForSmallIssueSizeMember 2019-01-01 2019-12-31 0000944695 srt:MaximumMember us-gaap:CommercialMortgageBackedSecuritiesMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:FairValueInputsLevel3Member thg:MeasurementInputDiscountForSmallIssueSizeMember 2019-01-01 2019-12-31 0000944695 srt:WeightedAverageMember us-gaap:CommercialMortgageBackedSecuritiesMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:FairValueInputsLevel3Member thg:MeasurementInputDiscountForSmallIssueSizeMember 2019-01-01 2019-12-31 0000944695 srt:MinimumMember us-gaap:CommercialMortgageBackedSecuritiesMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:FairValueInputsLevel3Member thg:MeasurementInputDiscountForSmallIssueSizeMember 2018-01-01 2018-12-31 0000944695 srt:MaximumMember us-gaap:CommercialMortgageBackedSecuritiesMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:FairValueInputsLevel3Member thg:MeasurementInputDiscountForSmallIssueSizeMember 2018-01-01 2018-12-31 0000944695 srt:WeightedAverageMember us-gaap:CommercialMortgageBackedSecuritiesMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:FairValueInputsLevel3Member thg:MeasurementInputDiscountForSmallIssueSizeMember 2018-01-01 2018-12-31 0000944695 us-gaap:CommercialMortgageBackedSecuritiesMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:FairValueInputsLevel3Member thg:MeasurementInputDiscountRateForAboveMarketCouponMember 2019-01-01 2019-12-31 0000944695 srt:WeightedAverageMember us-gaap:CommercialMortgageBackedSecuritiesMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:FairValueInputsLevel3Member thg:MeasurementInputDiscountRateForAboveMarketCouponMember 2019-01-01 2019-12-31 0000944695 us-gaap:CommercialMortgageBackedSecuritiesMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:FairValueInputsLevel3Member thg:MeasurementInputDiscountRateForAboveMarketCouponMember 2018-01-01 2018-12-31 0000944695 srt:WeightedAverageMember us-gaap:CommercialMortgageBackedSecuritiesMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:FairValueInputsLevel3Member thg:MeasurementInputDiscountRateForAboveMarketCouponMember 2018-01-01 2018-12-31 0000944695 us-gaap:CommercialMortgageBackedSecuritiesMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:FairValueInputsLevel3Member thg:MeasurementInputDiscountRateForLeaseStructureMember 2019-01-01 2019-12-31 0000944695 srt:WeightedAverageMember us-gaap:CommercialMortgageBackedSecuritiesMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:FairValueInputsLevel3Member thg:MeasurementInputDiscountRateForLeaseStructureMember 2019-01-01 2019-12-31 0000944695 us-gaap:CommercialMortgageBackedSecuritiesMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:FairValueInputsLevel3Member thg:MeasurementInputDiscountRateForLeaseStructureMember 2018-01-01 2018-12-31 0000944695 srt:WeightedAverageMember us-gaap:CommercialMortgageBackedSecuritiesMember us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:FairValueInputsLevel3Member thg:MeasurementInputDiscountRateForLeaseStructureMember 2018-01-01 2018-12-31 0000944695 srt:WeightedAverageMember us-gaap:FairValueInputsLevel3Member 2019-12-31 0000944695 srt:WeightedAverageMember us-gaap:FairValueInputsLevel3Member 2018-12-31 0000944695 thg:SeniorDebenturesUnsecuredMaturingOnAprilFifteenthTwoThousandTwentySixMember 2019-12-31 0000944695 thg:SeniorDebenturesUnsecuredMaturingOnAprilFifteenthTwoThousandTwentySixMember 2018-12-31 0000944695 thg:SeniorDebenturesUnsecuredMaturingOnOctoberFifteenthTwoThousandTwentyFiveMember 2019-12-31 0000944695 thg:SeniorDebenturesUnsecuredMaturingOnOctoberFifteenthTwoThousandTwentyFiveMember 2018-12-31 0000944695 thg:SubordinatedDebenturesUnsecuredMaturingMarchThirtyTwoThousandFiftyThreeMember 2019-12-31 0000944695 thg:SubordinatedDebenturesUnsecuredMaturingMarchThirtyTwoThousandFiftyThreeMember 2018-12-31 0000944695 thg:SubordinatedDebenturesMaturingFebruaryThirdTwoThousandTwentySevenMember 2019-12-31 0000944695 thg:SubordinatedDebenturesMaturingFebruaryThirdTwoThousandTwentySevenMember 2018-12-31 0000944695 thg:FederalHomeLoanBankBorrowingsSecuredMember 2018-12-31 0000944695 thg:SeniorDebenturesUnsecuredMaturingOnAprilFifteenthTwoThousandTwentySixMember 2019-01-01 2019-12-31 0000944695 thg:SeniorDebenturesUnsecuredMaturingOnAprilFifteenthTwoThousandTwentySixMember 2018-01-01 2018-12-31 0000944695 thg:SeniorDebenturesUnsecuredMaturingOnOctoberFifteenthTwoThousandTwentyFiveMember 2019-01-01 2019-12-31 0000944695 thg:SeniorDebenturesUnsecuredMaturingOnOctoberFifteenthTwoThousandTwentyFiveMember 2018-01-01 2018-12-31 0000944695 thg:SubordinatedDebenturesUnsecuredMaturingMarchThirtyTwoThousandFiftyThreeMember 2019-01-01 2019-12-31 0000944695 thg:SubordinatedDebenturesUnsecuredMaturingMarchThirtyTwoThousandFiftyThreeMember 2018-01-01 2018-12-31 0000944695 thg:SubordinatedDebenturesMaturingFebruaryThirdTwoThousandTwentySevenMember 2019-01-01 2019-12-31 0000944695 thg:SubordinatedDebenturesMaturingFebruaryThirdTwoThousandTwentySevenMember 2018-01-01 2018-12-31 0000944695 thg:SeriesBEightPointTwoZeroSevenPercentageSubordinatedDeferrableInterestDebenturesMember 2019-12-31 0000944695 thg:SeriesBEightPointTwoZeroSevenPercentageSubordinatedDeferrableInterestDebenturesMember 2018-12-31 0000944695 thg:SeriesBEightPointTwoZeroSevenPercentageSubordinatedDeferrableInterestDebenturesMember 2019-01-01 2019-12-31 0000944695 thg:SeriesBEightPointTwoZeroSevenPercentageSubordinatedDeferrableInterestDebenturesMember 2018-01-01 2018-12-31 0000944695 2019-01-02 2019-01-02 0000944695 2019-01-02 0000944695 thg:CollateralizedBorrowingProgramMember 2019-12-31 0000944695 thg:CollateralizedBorrowingProgramMember 2018-12-31 0000944695 srt:ScenarioForecastMember 2020-01-01 2020-12-31 0000944695 2005-01-01 0000944695 2004-12-31 2005-01-01 0000944695 country:US 2019-12-31 0000944695 us-gaap:QualifiedPlanMember country:US 2019-12-31 0000944695 us-gaap:QualifiedPlanMember country:US 2018-12-31 0000944695 us-gaap:QualifiedPlanMember country:US 2017-12-31 0000944695 us-gaap:NonqualifiedPlanMember country:US 2019-12-31 0000944695 us-gaap:NonqualifiedPlanMember country:US 2018-12-31 0000944695 us-gaap:NonqualifiedPlanMember country:US 2017-12-31 0000944695 country:US 2019-01-01 2019-12-31 0000944695 country:US 2018-01-01 2018-12-31 0000944695 country:US 2017-01-01 2017-12-31 0000944695 us-gaap:QualifiedPlanMember country:US 2019-01-01 2019-12-31 0000944695 us-gaap:QualifiedPlanMember country:US 2018-01-01 2018-12-31 0000944695 us-gaap:QualifiedPlanMember country:US 2017-01-01 2017-12-31 0000944695 us-gaap:NonqualifiedPlanMember country:US 2019-01-01 2019-12-31 0000944695 us-gaap:NonqualifiedPlanMember country:US 2018-01-01 2018-12-31 0000944695 us-gaap:NonqualifiedPlanMember country:US 2017-01-01 2017-12-31 0000944695 us-gaap:FixedIncomeSecuritiesMember country:US 2018-12-31 0000944695 us-gaap:FixedIncomeSecuritiesMember country:US 2019-12-31 0000944695 us-gaap:DefinedBenefitPlanEquitySecuritiesMember country:US 2019-12-31 0000944695 us-gaap:DefinedBenefitPlanEquitySecuritiesMember country:US 2018-12-31 0000944695 us-gaap:FixedMaturitiesMember country:US 2019-12-31 0000944695 us-gaap:MoneyMarketFundsMember country:US 2019-12-31 0000944695 thg:EquitySecuritiesDomesticMember country:US 2019-12-31 0000944695 thg:EquitySecuritiesInternationalMember country:US 2019-12-31 0000944695 us-gaap:FixedMaturitiesMember country:US 2018-12-31 0000944695 us-gaap:MoneyMarketFundsMember country:US 2018-12-31 0000944695 thg:EquitySecuritiesDomesticMember country:US 2018-12-31 0000944695 thg:EquitySecuritiesInternationalMember country:US 2018-12-31 0000944695 country:US 2018-12-31 0000944695 us-gaap:EstimateOfFairValueFairValueDisclosureMember thg:FixedMaturitiesSecuritiesMember country:US 2019-12-31 0000944695 us-gaap:EstimateOfFairValueFairValueDisclosureMember us-gaap:FairValueInputsLevel1Member thg:FixedMaturitiesSecuritiesMember country:US 2019-12-31 0000944695 us-gaap:EstimateOfFairValueFairValueDisclosureMember us-gaap:FairValueInputsLevel3Member thg:FixedMaturitiesSecuritiesMember country:US 2019-12-31 0000944695 us-gaap:EstimateOfFairValueFairValueDisclosureMember thg:FixedMaturitiesSecuritiesMember country:US 2018-12-31 0000944695 us-gaap:EstimateOfFairValueFairValueDisclosureMember us-gaap:FairValueInputsLevel1Member thg:FixedMaturitiesSecuritiesMember country:US 2018-12-31 0000944695 us-gaap:EstimateOfFairValueFairValueDisclosureMember us-gaap:FairValueInputsLevel3Member thg:FixedMaturitiesSecuritiesMember country:US 2018-12-31 0000944695 us-gaap:EstimateOfFairValueFairValueDisclosureMember us-gaap:MoneyMarketFundsMember country:US 2019-12-31 0000944695 us-gaap:EstimateOfFairValueFairValueDisclosureMember us-gaap:FairValueInputsLevel1Member us-gaap:MoneyMarketFundsMember country:US 2019-12-31 0000944695 us-gaap:EstimateOfFairValueFairValueDisclosureMember us-gaap:MoneyMarketFundsMember country:US 2018-12-31 0000944695 us-gaap:EstimateOfFairValueFairValueDisclosureMember us-gaap:FairValueInputsLevel1Member us-gaap:MoneyMarketFundsMember country:US 2018-12-31 0000944695 us-gaap:EstimateOfFairValueFairValueDisclosureMember thg:MutualFundInvestmentsMember country:US 2018-12-31 0000944695 us-gaap:EstimateOfFairValueFairValueDisclosureMember us-gaap:FairValueInputsLevel1Member thg:MutualFundInvestmentsMember country:US 2018-12-31 0000944695 us-gaap:EstimateOfFairValueFairValueDisclosureMember country:US 2019-12-31 0000944695 us-gaap:EstimateOfFairValueFairValueDisclosureMember us-gaap:FairValueInputsLevel1Member country:US 2019-12-31 0000944695 us-gaap:EstimateOfFairValueFairValueDisclosureMember us-gaap:FairValueInputsLevel3Member country:US 2019-12-31 0000944695 us-gaap:EstimateOfFairValueFairValueDisclosureMember country:US 2018-12-31 0000944695 us-gaap:EstimateOfFairValueFairValueDisclosureMember us-gaap:FairValueInputsLevel1Member country:US 2018-12-31 0000944695 us-gaap:EstimateOfFairValueFairValueDisclosureMember us-gaap:FairValueInputsLevel3Member country:US 2018-12-31 0000944695 us-gaap:FairValueMeasuredAtNetAssetValuePerShareMember 2019-12-31 0000944695 us-gaap:FairValueMeasuredAtNetAssetValuePerShareMember 2018-12-31 0000944695 us-gaap:FairValueMeasuredAtNetAssetValuePerShareMember thg:EquitySecuritiesDomesticMember 2019-12-31 0000944695 us-gaap:FairValueMeasuredAtNetAssetValuePerShareMember thg:EquitySecuritiesDomesticMember 2018-12-31 0000944695 us-gaap:FairValueMeasuredAtNetAssetValuePerShareMember thg:EquitySecuritiesInternationalMember 2018-12-31 0000944695 country:US 2017-12-31 0000944695 us-gaap:QualifiedPlanMember 2018-12-31 0000944695 us-gaap:QualifiedPlanMember 2017-12-31 0000944695 us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember us-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember 2019-01-01 2019-12-31 0000944695 us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember us-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember 2018-01-01 2018-12-31 0000944695 us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember us-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember 2017-01-01 2017-12-31 0000944695 us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember us-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember thg:ChaucerMember 2019-01-01 2019-12-31 0000944695 us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember us-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember thg:ChaucerMember 2018-01-01 2018-12-31 0000944695 us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember us-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember thg:ChaucerMember 2017-01-01 2017-12-31 0000944695 us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember us-gaap:AccumulatedDefinedBenefitPlansAdjustmentMember 2019-01-01 2019-12-31 0000944695 us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember us-gaap:AccumulatedDefinedBenefitPlansAdjustmentMember 2018-01-01 2018-12-31 0000944695 us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember us-gaap:AccumulatedDefinedBenefitPlansAdjustmentMember 2017-01-01 2017-12-31 0000944695 us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember thg:CurrencyTranslationObligationRecognizedWithSaleMember thg:ChaucerMember 2019-01-01 2019-12-31 0000944695 us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember thg:CurrencyTranslationObligationRecognizedWithSaleMember thg:ChaucerMember 2018-01-01 2018-12-31 0000944695 us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember 2019-01-01 2019-12-31 0000944695 us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember 2018-01-01 2018-12-31 0000944695 us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember 2017-01-01 2017-12-31 0000944695 thg:HanoverInsuranceGroup2014LongTermIncentivePlanMember 2014-05-20 0000944695 thg:StockOptionsAndStockAppreciationRightsMember 2014-05-19 2014-05-20 0000944695 2014-05-19 2014-05-20 0000944695 thg:HanoverInsuranceGroup2014LongTermIncentivePlanMember 2019-12-31 0000944695 us-gaap:EmployeeStockMember 2014-05-20 0000944695 us-gaap:EmployeeStockMember 2019-12-31 0000944695 us-gaap:ShareBasedCompensationAwardTrancheThreeMember 2019-01-01 2019-12-31 0000944695 us-gaap:ShareBasedCompensationAwardTrancheThreeMember 2018-01-01 2018-12-31 0000944695 us-gaap:ShareBasedCompensationAwardTrancheThreeMember 2017-01-01 2017-12-31 0000944695 2019-01-01 2019-01-31 0000944695 thg:HanoverInsuranceGroup2014LongTermIncentivePlanMember 2019-01-01 2019-12-31 0000944695 thg:HanoverInsuranceGroup2014LongTermIncentivePlanMember 2018-01-01 2018-12-31 0000944695 thg:HanoverInsuranceGroup2014LongTermIncentivePlanMember 2017-01-01 2017-12-31 0000944695 thg:ExercisePriceRangeOneMember 2019-01-01 2019-12-31 0000944695 thg:ExercisePriceRangeTwoMember 2019-01-01 2019-12-31 0000944695 thg:ExercisePriceRangeThreeMember 2019-01-01 2019-12-31 0000944695 thg:ExercisePriceRangeFourMember 2019-01-01 2019-12-31 0000944695 thg:ExercisePriceRangeFiveMember 2019-01-01 2019-12-31 0000944695 thg:ExercisePriceRangeSixMember 2019-01-01 2019-12-31 0000944695 thg:ExercisePriceRangeOneMember 2019-12-31 0000944695 thg:ExercisePriceRangeTwoMember 2019-12-31 0000944695 thg:ExercisePriceRangeThreeMember 2019-12-31 0000944695 thg:ExercisePriceRangeFourMember 2019-12-31 0000944695 thg:ExercisePriceRangeFiveMember 2019-12-31 0000944695 thg:ExercisePriceRangeSixMember 2019-12-31 0000944695 us-gaap:EmployeeStockOptionMember 2019-01-01 2019-12-31 0000944695 srt:MinimumMember us-gaap:EmployeeStockOptionMember 2018-01-01 2018-12-31 0000944695 srt:MaximumMember us-gaap:EmployeeStockOptionMember 2018-01-01 2018-12-31 0000944695 srt:MinimumMember us-gaap:EmployeeStockOptionMember 2017-01-01 2017-12-31 0000944695 srt:MaximumMember us-gaap:EmployeeStockOptionMember 2017-01-01 2017-12-31 0000944695 us-gaap:EmployeeStockOptionMember 2018-01-01 2018-12-31 0000944695 us-gaap:EmployeeStockOptionMember 2017-01-01 2017-12-31 0000944695 srt:MinimumMember us-gaap:EmployeeStockOptionMember 2019-01-01 2019-12-31 0000944695 srt:MaximumMember us-gaap:EmployeeStockOptionMember 2019-01-01 2019-12-31 0000944695 us-gaap:EmployeeStockOptionMember 2019-12-31 0000944695 thg:MarketBasedRestrictedStockUnitsMember 2019-01-01 2019-12-31 0000944695 thg:MarketBasedRestrictedStockUnitsMember 2018-01-01 2018-12-31 0000944695 thg:MarketBasedRestrictedStockUnitsMember 2017-01-01 2017-12-31 0000944695 thg:TimeBasedRestrictedStockUnitsMember 2019-01-01 2019-12-31 0000944695 thg:TimeBasedRestrictedStockUnitsMember 2018-01-01 2018-12-31 0000944695 thg:TimeBasedRestrictedStockUnitsMember 2017-01-01 2017-12-31 0000944695 thg:TimeBasedRestrictedStockUnitsMember 2018-12-31 0000944695 thg:TimeBasedRestrictedStockUnitsMember 2017-12-31 0000944695 thg:TimeBasedRestrictedStockUnitsMember 2016-12-31 0000944695 thg:TimeBasedRestrictedStockUnitsMember 2019-12-31 0000944695 thg:PerformanceBasedAndMarketBasedRestrictedStockUnitsMember 2018-12-31 0000944695 thg:PerformanceBasedAndMarketBasedRestrictedStockUnitsMember 2017-12-31 0000944695 thg:PerformanceBasedAndMarketBasedRestrictedStockUnitsMember 2016-12-31 0000944695 thg:PerformanceBasedAndMarketBasedRestrictedStockUnitsMember 2019-01-01 2019-12-31 0000944695 thg:PerformanceBasedAndMarketBasedRestrictedStockUnitsMember 2018-01-01 2018-12-31 0000944695 thg:PerformanceBasedAndMarketBasedRestrictedStockUnitsMember 2017-01-01 2017-12-31 0000944695 thg:PerformanceBasedAndMarketBasedRestrictedStockUnitsMember 2019-12-31 0000944695 thg:PerformanceBasedRestrictedStockUnitMember 2018-01-01 2018-12-31 0000944695 thg:PerformanceAndMarketBasedRestrictedStockUnitsMember srt:MinimumMember thg:KeyExecutivesMember 2019-01-01 2019-12-31 0000944695 thg:PerformanceAndMarketBasedRestrictedStockUnitsMember srt:MaximumMember thg:KeyExecutivesMember 2019-01-01 2019-12-31 0000944695 thg:MarketBasedRestrictedStockUnitsMember thg:ShareUnitsGrantedIn2016Member 2019-01-01 2019-12-31 0000944695 thg:MarketBasedRestrictedStockUnitsMember thg:ShareUnitsGrantedIn2017Member 2019-01-01 2019-12-31 0000944695 thg:MarketBasedRestrictedStockUnitsMember thg:ShareUnitsGrantedIn2015Member 2018-01-01 2018-12-31 0000944695 thg:MarketBasedRestrictedStockUnitsMember thg:ShareUnitsGrantedIn2014Member 2017-01-01 2017-12-31 0000944695 thg:PerformanceBasedRestrictedStockUnitMember 2019-01-01 2019-12-31 0000944695 thg:PerformanceBasedRestrictedStockUnitMember srt:MinimumMember 2019-01-01 2019-12-31 0000944695 thg:PerformanceBasedRestrictedStockUnitMember srt:MaximumMember 2019-01-01 2019-12-31 0000944695 thg:RestrictedStockAndRestrictedStockUnitsMember 2019-01-01 2019-12-31 0000944695 thg:RestrictedStockAndRestrictedStockUnitsMember 2018-01-01 2018-12-31 0000944695 thg:RestrictedStockAndRestrictedStockUnitsMember 2017-01-01 2017-12-31 0000944695 thg:PerformanceAndMarketBasedRestrictedStockUnitsMember 2019-01-01 2019-12-31 0000944695 thg:PerformanceAndMarketBasedRestrictedStockUnitsMember 2018-01-01 2018-12-31 0000944695 thg:PerformanceAndMarketBasedRestrictedStockUnitsMember 2017-01-01 2017-12-31 0000944695 thg:PerformanceBasedRestrictedStockUnitMember 2017-01-01 2017-12-31 0000944695 us-gaap:RestrictedStockUnitsRSUMember 2019-12-31 0000944695 us-gaap:RestrictedStockUnitsRSUMember 2019-01-01 2019-12-31 0000944695 thg:PerformanceAndMarketBasedRestrictedStockUnitsMember 2019-12-31 0000944695 thg:UnvestedRestrictedStockUnitsAndPerformanceAndMarketBasedRestrictedStockUnitsMember 2019-12-31 0000944695 thg:UnvestedRestrictedStockUnitsAndPerformanceAndMarketBasedRestrictedStockUnitsMember 2019-01-01 2019-12-31 0000944695 us-gaap:EmployeeStockOptionMember 2019-01-01 2019-12-31 0000944695 us-gaap:EmployeeStockOptionMember 2018-01-01 2018-12-31 0000944695 us-gaap:EmployeeStockOptionMember 2017-01-01 2017-12-31 0000944695 thg:NonVestedStockGrantsAndOtherMember 2019-01-01 2019-12-31 0000944695 thg:NonVestedStockGrantsAndOtherMember 2018-01-01 2018-12-31 0000944695 thg:NonVestedStockGrantsAndOtherMember 2017-01-01 2017-12-31 0000944695 thg:AcceleratedShareRepurchaseProgramMember srt:MaximumMember 2019-12-31 0000944695 thg:AcceleratedShareRepurchaseProgramMember 2019-12-05 2019-12-05 0000944695 thg:DecemberTwoThousandEighteenAndJuneTwoThousandNineteenAcceleratedShareRepurchaseProgramMember 2019-01-01 2019-12-31 0000944695 thg:DecemberTwoThousandNineteenAcceleratedShareRepurchaseAgreementMember 2019-12-09 0000944695 thg:DecemberTwoThousandNineteenAcceleratedShareRepurchaseAgreementMember 2019-12-09 2019-12-09 0000944695 stpr:NH 2019-01-01 2019-12-31 0000944695 stpr:NH 2019-12-31 0000944695 stpr:NH 2018-12-31 0000944695 stpr:NH 2017-12-31 0000944695 stpr:NH 2018-01-01 2018-12-31 0000944695 stpr:NH 2017-01-01 2017-12-31 0000944695 stpr:NH srt:MaximumMember srt:ScenarioForecastMember 2020-05-31 0000944695 stpr:NH srt:MaximumMember us-gaap:SubsequentEventMember 2020-01-01 0000944695 stpr:NH srt:ScenarioForecastMember 2020-05-31 0000944695 stpr:MI 2019-01-01 2019-12-31 0000944695 stpr:MI 2019-12-31 0000944695 stpr:MI 2018-12-31 0000944695 stpr:MI 2017-12-31 0000944695 stpr:MI srt:MaximumMember 2019-12-31 thg:segment 0000944695 thg:CommercialLinesSegmentMember 2019-01-01 2019-12-31 0000944695 thg:CommercialLinesSegmentMember 2018-01-01 2018-12-31 0000944695 thg:CommercialLinesSegmentMember 2017-01-01 2017-12-31 0000944695 thg:PersonalLinesSegmentMember 2019-01-01 2019-12-31 0000944695 thg:PersonalLinesSegmentMember 2018-01-01 2018-12-31 0000944695 thg:PersonalLinesSegmentMember 2017-01-01 2017-12-31 0000944695 us-gaap:AllOtherSegmentsMember 2019-01-01 2019-12-31 0000944695 us-gaap:AllOtherSegmentsMember 2018-01-01 2018-12-31 0000944695 us-gaap:AllOtherSegmentsMember 2017-01-01 2017-12-31 0000944695 thg:PropertyAndCasualtyMember 2019-12-31 0000944695 thg:PropertyAndCasualtyMember 2018-12-31 0000944695 us-gaap:DiscontinuedOperationsDisposedOfBySaleMember 2019-12-31 0000944695 us-gaap:DiscontinuedOperationsDisposedOfBySaleMember 2018-12-31 0000944695 thg:MichiganCatastrophicClaimsAssociationMember 2019-01-01 2019-12-31 0000944695 thg:MichiganCatastrophicClaimsAssociationMember 2018-01-01 2018-12-31 0000944695 thg:MichiganCatastrophicClaimsAssociationMember 2017-01-01 2017-12-31 0000944695 thg:MichiganCatastrophicClaimsAssociationMember thg:ReinsuranceReceivableMember us-gaap:ReinsurerConcentrationRiskMember 2019-01-01 2019-12-31 0000944695 thg:MichiganCatastrophicClaimsAssociationMember 2019-12-31 0000944695 thg:MichiganCatastrophicClaimsAssociationMember 2018-12-31 0000944695 thg:CommercialMultiplePerilLineMember thg:CommercialLinesSegmentMember 2019-01-01 2019-12-31 0000944695 thg:WorkersCompensationLineMember thg:CommercialLinesSegmentMember 2019-01-01 2019-12-31 0000944695 thg:CommercialAutomobileLineMember thg:CommercialLinesSegmentMember 2019-01-01 2019-12-31 0000944695 thg:HanoverProgramsLineMember thg:CommercialLinesSegmentMember 2019-01-01 2019-12-31 0000944695 thg:CommercialLinesSegmentMember us-gaap:GeneralLiabilityMember 2019-01-01 2019-12-31 0000944695 thg:MarineLineMember thg:CommercialLinesSegmentMember 2019-01-01 2019-12-31 0000944695 thg:SuretyLineMember thg:CommercialLinesSegmentMember 2019-01-01 2019-12-31 0000944695 thg:UmbrellaLineMember thg:CommercialLinesSegmentMember 2019-01-01 2019-12-31 0000944695 thg:OtherLinesMember thg:CommercialLinesSegmentMember 2019-01-01 2019-12-31 0000944695 thg:OtherCommercialLinesMember thg:CommercialLinesSegmentMember 2019-01-01 2019-12-31 0000944695 thg:PersonalAutomobileLinesMember thg:PersonalLinesSegmentMember 2019-01-01 2019-12-31 0000944695 thg:HomeownersAndOtherPersonalLinesMember thg:PersonalLinesSegmentMember 2019-01-01 2019-12-31 0000944695 thg:CommercialMultiplePerilLineMember thg:CommercialLinesSegmentMember 2018-01-01 2018-12-31 0000944695 thg:WorkersCompensationLineMember thg:CommercialLinesSegmentMember 2018-01-01 2018-12-31 0000944695 thg:CommercialAutomobileLineMember thg:CommercialLinesSegmentMember 2018-01-01 2018-12-31 0000944695 thg:HanoverProgramsLineMember thg:CommercialLinesSegmentMember 2018-01-01 2018-12-31 0000944695 thg:CommercialLinesSegmentMember us-gaap:GeneralLiabilityMember 2018-01-01 2018-12-31 0000944695 thg:MarineLineMember thg:CommercialLinesSegmentMember 2018-01-01 2018-12-31 0000944695 thg:SuretyLineMember thg:CommercialLinesSegmentMember 2018-01-01 2018-12-31 0000944695 thg:UmbrellaLineMember thg:CommercialLinesSegmentMember 2018-01-01 2018-12-31 0000944695 thg:OtherLinesMember thg:CommercialLinesSegmentMember 2018-01-01 2018-12-31 0000944695 thg:OtherCommercialLinesMember thg:CommercialLinesSegmentMember 2018-01-01 2018-12-31 0000944695 thg:PersonalAutomobileLinesMember thg:PersonalLinesSegmentMember 2018-01-01 2018-12-31 0000944695 thg:HomeownersAndOtherPersonalLinesMember thg:PersonalLinesSegmentMember 2018-01-01 2018-12-31 0000944695 thg:CommercialMultiplePerilLineMember thg:CommercialLinesSegmentMember 2017-01-01 2017-12-31 0000944695 thg:WorkersCompensationLineMember thg:CommercialLinesSegmentMember 2017-01-01 2017-12-31 0000944695 thg:CommercialAutomobileLineMember thg:CommercialLinesSegmentMember 2017-01-01 2017-12-31 0000944695 thg:HanoverProgramsLineMember thg:CommercialLinesSegmentMember 2017-01-01 2017-12-31 0000944695 thg:CommercialLinesSegmentMember us-gaap:GeneralLiabilityMember 2017-01-01 2017-12-31 0000944695 thg:MarineLineMember thg:CommercialLinesSegmentMember 2017-01-01 2017-12-31 0000944695 thg:SuretyLineMember thg:CommercialLinesSegmentMember 2017-01-01 2017-12-31 0000944695 thg:UmbrellaLineMember thg:CommercialLinesSegmentMember 2017-01-01 2017-12-31 0000944695 thg:OtherLinesMember thg:CommercialLinesSegmentMember 2017-01-01 2017-12-31 0000944695 thg:OtherCommercialLinesMember thg:CommercialLinesSegmentMember 2017-01-01 2017-12-31 0000944695 thg:PersonalAutomobileLinesMember thg:PersonalLinesSegmentMember 2017-01-01 2017-12-31 0000944695 thg:HomeownersAndOtherPersonalLinesMember thg:PersonalLinesSegmentMember 2017-01-01 2017-12-31 0000944695 thg:WorkersCompensationLineMember thg:ShortDurationInsuranceContractsAccidentYears2015To2018Member 2019-01-01 2019-12-31 0000944695 thg:PersonalAutomobileLinesMember thg:ShortDurationInsuranceContractsAccidentYears2016To2017Member 2019-01-01 2019-12-31 0000944695 thg:WorkersCompensationLineMember thg:ShortDurationInsuranceContractsAccidentYears2015To2017Member 2018-01-01 2018-12-31 0000944695 thg:CommercialAutomobileLineMember thg:ShortDurationInsuranceContractsAccidentYears20142016And2017Member 2018-01-01 2018-12-31 0000944695 thg:OtherCommercialLinesMember thg:ShortDurationInsuranceContractsAccidentYears20142016And2017Member 2018-01-01 2018-12-31 0000944695 thg:CommercialMultiplePerilLineMember thg:CommercialAndOtherLinesMember 2019-12-31 0000944695 thg:WorkersCompensationLineMember thg:CommercialAndOtherLinesMember 2019-12-31 0000944695 thg:CommercialAutomobileLineMember thg:CommercialAndOtherLinesMember 2019-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember thg:CommercialAndOtherLinesMember 2019-12-31 0000944695 thg:GeneralLiabilityClaimsMadeMember thg:CommercialAndOtherLinesMember 2019-12-31 0000944695 thg:OtherLinesMember thg:CommercialAndOtherLinesMember 2019-12-31 0000944695 thg:CommercialAndOtherLinesMember 2019-12-31 0000944695 thg:PersonalAutomobileLinesMember thg:PersonalLinesSegmentMember 2019-12-31 0000944695 thg:HomeownersMember thg:PersonalLinesSegmentMember 2019-12-31 0000944695 thg:OtherPersonalLinesMember thg:PersonalLinesSegmentMember 2019-12-31 0000944695 thg:PersonalLinesSegmentMember 2019-12-31 0000944695 thg:CommercialMultiplePerilLineMember thg:CommercialAndOtherLinesMember 2018-12-31 0000944695 thg:WorkersCompensationLineMember thg:CommercialAndOtherLinesMember 2018-12-31 0000944695 thg:CommercialAutomobileLineMember thg:CommercialAndOtherLinesMember 2018-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember thg:CommercialAndOtherLinesMember 2018-12-31 0000944695 thg:GeneralLiabilityClaimsMadeMember thg:CommercialAndOtherLinesMember 2018-12-31 0000944695 thg:OtherLinesMember thg:CommercialAndOtherLinesMember 2018-12-31 0000944695 thg:CommercialAndOtherLinesMember 2018-12-31 0000944695 thg:PersonalAutomobileLinesMember thg:PersonalLinesSegmentMember 2018-12-31 0000944695 thg:HomeownersMember thg:PersonalLinesSegmentMember 2018-12-31 0000944695 thg:OtherPersonalLinesMember thg:PersonalLinesSegmentMember 2018-12-31 0000944695 thg:PersonalLinesSegmentMember 2018-12-31 0000944695 us-gaap:AsbestosIssueMember 2019-12-31 0000944695 us-gaap:AsbestosIssueMember 2018-12-31 0000944695 country:US 2017-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2013Member thg:CommercialMultiplePerilLineMember 2013-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2013Member thg:CommercialMultiplePerilLineMember 2014-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member thg:CommercialMultiplePerilLineMember 2014-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2013Member thg:CommercialMultiplePerilLineMember 2015-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member thg:CommercialMultiplePerilLineMember 2015-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2015Member thg:CommercialMultiplePerilLineMember 2015-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2013Member thg:CommercialMultiplePerilLineMember 2016-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member thg:CommercialMultiplePerilLineMember 2016-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2015Member thg:CommercialMultiplePerilLineMember 2016-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2016Member thg:CommercialMultiplePerilLineMember 2016-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2013Member thg:CommercialMultiplePerilLineMember 2017-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member thg:CommercialMultiplePerilLineMember 2017-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2015Member thg:CommercialMultiplePerilLineMember 2017-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2016Member thg:CommercialMultiplePerilLineMember 2017-12-31 0000944695 us-gaap:ShortDurationInsuranceContractsAccidentYear2017Member thg:CommercialMultiplePerilLineMember 2017-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2013Member thg:CommercialMultiplePerilLineMember 2018-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member thg:CommercialMultiplePerilLineMember 2018-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2015Member thg:CommercialMultiplePerilLineMember 2018-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2016Member thg:CommercialMultiplePerilLineMember 2018-12-31 0000944695 us-gaap:ShortDurationInsuranceContractsAccidentYear2017Member thg:CommercialMultiplePerilLineMember 2018-12-31 0000944695 us-gaap:ShortDurationInsuranceContractsAccidentYear2018Member thg:CommercialMultiplePerilLineMember 2018-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2013Member thg:CommercialMultiplePerilLineMember 2019-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member thg:CommercialMultiplePerilLineMember 2019-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2015Member thg:CommercialMultiplePerilLineMember 2019-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2016Member thg:CommercialMultiplePerilLineMember 2019-12-31 0000944695 us-gaap:ShortDurationInsuranceContractsAccidentYear2017Member thg:CommercialMultiplePerilLineMember 2019-12-31 0000944695 us-gaap:ShortDurationInsuranceContractsAccidentYear2018Member thg:CommercialMultiplePerilLineMember 2019-12-31 0000944695 thg:ShortDurationInsuranceContractsAccidentYear2019Member thg:CommercialMultiplePerilLineMember 2019-12-31 0000944695 thg:CommercialMultiplePerilLineMember 2019-12-31 thg:claim 0000944695 thg:ShortDurationInsuranceContractsAccidentYears2013To2019Member thg:CommercialMultiplePerilLineMember 2019-12-31 0000944695 thg:ShortDurationInsuranceContractsAccidentYears2012AndPriorMember thg:CommercialMultiplePerilLineMember 2019-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2010Member thg:WorkersCompensationLineMember 2010-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2010Member thg:WorkersCompensationLineMember 2011-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2011Member thg:WorkersCompensationLineMember 2011-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2010Member thg:WorkersCompensationLineMember 2012-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2011Member thg:WorkersCompensationLineMember 2012-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2012Member thg:WorkersCompensationLineMember 2012-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2010Member thg:WorkersCompensationLineMember 2013-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2011Member thg:WorkersCompensationLineMember 2013-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2012Member thg:WorkersCompensationLineMember 2013-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2013Member thg:WorkersCompensationLineMember 2013-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2010Member thg:WorkersCompensationLineMember 2014-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2011Member thg:WorkersCompensationLineMember 2014-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2012Member thg:WorkersCompensationLineMember 2014-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2013Member thg:WorkersCompensationLineMember 2014-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member thg:WorkersCompensationLineMember 2014-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2010Member thg:WorkersCompensationLineMember 2015-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2011Member thg:WorkersCompensationLineMember 2015-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2012Member thg:WorkersCompensationLineMember 2015-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2013Member thg:WorkersCompensationLineMember 2015-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member thg:WorkersCompensationLineMember 2015-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2015Member thg:WorkersCompensationLineMember 2015-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2010Member thg:WorkersCompensationLineMember 2016-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2011Member thg:WorkersCompensationLineMember 2016-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2012Member thg:WorkersCompensationLineMember 2016-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2013Member thg:WorkersCompensationLineMember 2016-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member thg:WorkersCompensationLineMember 2016-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2015Member thg:WorkersCompensationLineMember 2016-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2016Member thg:WorkersCompensationLineMember 2016-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2010Member thg:WorkersCompensationLineMember 2017-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2011Member thg:WorkersCompensationLineMember 2017-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2012Member thg:WorkersCompensationLineMember 2017-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2013Member thg:WorkersCompensationLineMember 2017-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member thg:WorkersCompensationLineMember 2017-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2015Member thg:WorkersCompensationLineMember 2017-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2016Member thg:WorkersCompensationLineMember 2017-12-31 0000944695 us-gaap:ShortDurationInsuranceContractsAccidentYear2017Member thg:WorkersCompensationLineMember 2017-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2010Member thg:WorkersCompensationLineMember 2018-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2011Member thg:WorkersCompensationLineMember 2018-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2012Member thg:WorkersCompensationLineMember 2018-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2013Member thg:WorkersCompensationLineMember 2018-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member thg:WorkersCompensationLineMember 2018-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2015Member thg:WorkersCompensationLineMember 2018-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2016Member thg:WorkersCompensationLineMember 2018-12-31 0000944695 us-gaap:ShortDurationInsuranceContractsAccidentYear2017Member thg:WorkersCompensationLineMember 2018-12-31 0000944695 us-gaap:ShortDurationInsuranceContractsAccidentYear2018Member thg:WorkersCompensationLineMember 2018-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2010Member thg:WorkersCompensationLineMember 2019-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2011Member thg:WorkersCompensationLineMember 2019-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2012Member thg:WorkersCompensationLineMember 2019-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2013Member thg:WorkersCompensationLineMember 2019-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member thg:WorkersCompensationLineMember 2019-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2015Member thg:WorkersCompensationLineMember 2019-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2016Member thg:WorkersCompensationLineMember 2019-12-31 0000944695 us-gaap:ShortDurationInsuranceContractsAccidentYear2017Member thg:WorkersCompensationLineMember 2019-12-31 0000944695 us-gaap:ShortDurationInsuranceContractsAccidentYear2018Member thg:WorkersCompensationLineMember 2019-12-31 0000944695 thg:ShortDurationInsuranceContractsAccidentYear2019Member thg:WorkersCompensationLineMember 2019-12-31 0000944695 thg:WorkersCompensationLineMember 2019-12-31 0000944695 thg:ShortDurationInsuranceContractsAccidentYears2010To2019Member thg:WorkersCompensationLineMember 2019-12-31 0000944695 thg:ShortDurationInsuranceContractsAccidentYears2009AndPriorMember thg:WorkersCompensationLineMember 2019-12-31 0000944695 thg:CommercialAutomobileLineMember us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member 2013-12-31 0000944695 thg:CommercialAutomobileLineMember us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member 2015-12-31 0000944695 thg:CommercialAutomobileLineMember us-gaap:ShortdurationInsuranceContractsAccidentYear2015Member 2015-12-31 0000944695 thg:CommercialAutomobileLineMember us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member 2016-12-31 0000944695 thg:CommercialAutomobileLineMember us-gaap:ShortdurationInsuranceContractsAccidentYear2015Member 2016-12-31 0000944695 thg:CommercialAutomobileLineMember us-gaap:ShortdurationInsuranceContractsAccidentYear2016Member 2016-12-31 0000944695 thg:CommercialAutomobileLineMember us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member 2017-12-31 0000944695 thg:CommercialAutomobileLineMember us-gaap:ShortdurationInsuranceContractsAccidentYear2015Member 2017-12-31 0000944695 thg:CommercialAutomobileLineMember us-gaap:ShortdurationInsuranceContractsAccidentYear2016Member 2017-12-31 0000944695 thg:CommercialAutomobileLineMember us-gaap:ShortDurationInsuranceContractsAccidentYear2017Member 2017-12-31 0000944695 thg:CommercialAutomobileLineMember us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member 2018-12-31 0000944695 thg:CommercialAutomobileLineMember us-gaap:ShortdurationInsuranceContractsAccidentYear2015Member 2018-12-31 0000944695 thg:CommercialAutomobileLineMember us-gaap:ShortdurationInsuranceContractsAccidentYear2016Member 2018-12-31 0000944695 thg:CommercialAutomobileLineMember us-gaap:ShortDurationInsuranceContractsAccidentYear2017Member 2018-12-31 0000944695 thg:CommercialAutomobileLineMember us-gaap:ShortDurationInsuranceContractsAccidentYear2018Member 2018-12-31 0000944695 thg:CommercialAutomobileLineMember us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member 2019-12-31 0000944695 thg:CommercialAutomobileLineMember us-gaap:ShortdurationInsuranceContractsAccidentYear2015Member 2019-12-31 0000944695 thg:CommercialAutomobileLineMember us-gaap:ShortdurationInsuranceContractsAccidentYear2016Member 2019-12-31 0000944695 thg:CommercialAutomobileLineMember us-gaap:ShortDurationInsuranceContractsAccidentYear2017Member 2019-12-31 0000944695 thg:CommercialAutomobileLineMember us-gaap:ShortDurationInsuranceContractsAccidentYear2018Member 2019-12-31 0000944695 thg:CommercialAutomobileLineMember thg:ShortDurationInsuranceContractsAccidentYear2019Member 2019-12-31 0000944695 thg:CommercialAutomobileLineMember 2019-12-31 0000944695 thg:CommercialAutomobileLineMember thg:ShortDurationInsuranceContractsAccidentYears2014To2019Member 2019-12-31 0000944695 thg:CommercialAutomobileLineMember thg:ShortDurationInsuranceContractsAccidentYears2013AndPriorMember 2019-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2011Member 2011-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2011Member 2012-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2012Member 2012-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2011Member 2013-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2012Member 2013-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2013Member 2013-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2011Member 2014-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2012Member 2014-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2013Member 2014-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member 2014-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2011Member 2015-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2012Member 2015-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2013Member 2015-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member 2015-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2015Member 2015-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2011Member 2016-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2012Member 2016-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2013Member 2016-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member 2016-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2015Member 2016-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2016Member 2016-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2011Member 2017-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2012Member 2017-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2013Member 2017-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member 2017-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2015Member 2017-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2016Member 2017-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortDurationInsuranceContractsAccidentYear2017Member 2017-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2011Member 2018-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2012Member 2018-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2013Member 2018-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member 2018-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2015Member 2018-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2016Member 2018-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortDurationInsuranceContractsAccidentYear2017Member 2018-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortDurationInsuranceContractsAccidentYear2018Member 2018-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2011Member 2019-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2012Member 2019-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2013Member 2019-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member 2019-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2015Member 2019-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortdurationInsuranceContractsAccidentYear2016Member 2019-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortDurationInsuranceContractsAccidentYear2017Member 2019-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember us-gaap:ShortDurationInsuranceContractsAccidentYear2018Member 2019-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember thg:ShortDurationInsuranceContractsAccidentYear2019Member 2019-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember 2019-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember thg:ShortDurationInsuranceContractsAccidentYears2011To2019Member 2019-12-31 0000944695 thg:GeneralLiabilityAndUmbrellaOccurrenceMember thg:ShortDurationInsuranceContractsAccidentYears2010AndPriorMember 2019-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member thg:GeneralLiabilityClaimsMadeMember 2014-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member thg:GeneralLiabilityClaimsMadeMember 2015-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2015Member thg:GeneralLiabilityClaimsMadeMember 2015-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member thg:GeneralLiabilityClaimsMadeMember 2016-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2015Member thg:GeneralLiabilityClaimsMadeMember 2016-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2016Member thg:GeneralLiabilityClaimsMadeMember 2016-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member thg:GeneralLiabilityClaimsMadeMember 2017-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2015Member thg:GeneralLiabilityClaimsMadeMember 2017-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2016Member thg:GeneralLiabilityClaimsMadeMember 2017-12-31 0000944695 us-gaap:ShortDurationInsuranceContractsAccidentYear2017Member thg:GeneralLiabilityClaimsMadeMember 2017-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member thg:GeneralLiabilityClaimsMadeMember 2018-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2015Member thg:GeneralLiabilityClaimsMadeMember 2018-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2016Member thg:GeneralLiabilityClaimsMadeMember 2018-12-31 0000944695 us-gaap:ShortDurationInsuranceContractsAccidentYear2017Member thg:GeneralLiabilityClaimsMadeMember 2018-12-31 0000944695 us-gaap:ShortDurationInsuranceContractsAccidentYear2018Member thg:GeneralLiabilityClaimsMadeMember 2018-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member thg:GeneralLiabilityClaimsMadeMember 2019-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2015Member thg:GeneralLiabilityClaimsMadeMember 2019-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2016Member thg:GeneralLiabilityClaimsMadeMember 2019-12-31 0000944695 us-gaap:ShortDurationInsuranceContractsAccidentYear2017Member thg:GeneralLiabilityClaimsMadeMember 2019-12-31 0000944695 us-gaap:ShortDurationInsuranceContractsAccidentYear2018Member thg:GeneralLiabilityClaimsMadeMember 2019-12-31 0000944695 thg:ShortDurationInsuranceContractsAccidentYear2019Member thg:GeneralLiabilityClaimsMadeMember 2019-12-31 0000944695 thg:GeneralLiabilityClaimsMadeMember 2019-12-31 0000944695 thg:ShortDurationInsuranceContractsAccidentYears2014To2019Member thg:GeneralLiabilityClaimsMadeMember 2019-12-31 0000944695 thg:ShortDurationInsuranceContractsAccidentYears2013AndPriorMember thg:GeneralLiabilityClaimsMadeMember 2019-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2015Member thg:PersonalAutomobileLinesMember 2015-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2015Member thg:PersonalAutomobileLinesMember 2016-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2016Member thg:PersonalAutomobileLinesMember 2016-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2015Member thg:PersonalAutomobileLinesMember 2017-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2016Member thg:PersonalAutomobileLinesMember 2017-12-31 0000944695 us-gaap:ShortDurationInsuranceContractsAccidentYear2017Member thg:PersonalAutomobileLinesMember 2017-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2015Member thg:PersonalAutomobileLinesMember 2018-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2016Member thg:PersonalAutomobileLinesMember 2018-12-31 0000944695 us-gaap:ShortDurationInsuranceContractsAccidentYear2017Member thg:PersonalAutomobileLinesMember 2018-12-31 0000944695 us-gaap:ShortDurationInsuranceContractsAccidentYear2018Member thg:PersonalAutomobileLinesMember 2018-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2015Member thg:PersonalAutomobileLinesMember 2019-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2016Member thg:PersonalAutomobileLinesMember 2019-12-31 0000944695 us-gaap:ShortDurationInsuranceContractsAccidentYear2017Member thg:PersonalAutomobileLinesMember 2019-12-31 0000944695 us-gaap:ShortDurationInsuranceContractsAccidentYear2018Member thg:PersonalAutomobileLinesMember 2019-12-31 0000944695 thg:ShortDurationInsuranceContractsAccidentYear2019Member thg:PersonalAutomobileLinesMember 2019-12-31 0000944695 thg:PersonalAutomobileLinesMember 2019-12-31 0000944695 thg:PersonalAutomobileLinesMember thg:ShortDurationInsuranceContractsAccidentYears2015To2019Member 2019-12-31 0000944695 thg:PersonalAutomobileLinesMember thg:ShortDurationInsuranceContractsAccidentYears2014AndPriorMember 2019-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2016Member thg:HomeownersMember 2016-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2016Member thg:HomeownersMember 2017-12-31 0000944695 us-gaap:ShortDurationInsuranceContractsAccidentYear2017Member thg:HomeownersMember 2017-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2016Member thg:HomeownersMember 2018-12-31 0000944695 us-gaap:ShortDurationInsuranceContractsAccidentYear2017Member thg:HomeownersMember 2018-12-31 0000944695 us-gaap:ShortDurationInsuranceContractsAccidentYear2018Member thg:HomeownersMember 2018-12-31 0000944695 us-gaap:ShortdurationInsuranceContractsAccidentYear2016Member thg:HomeownersMember 2019-12-31 0000944695 us-gaap:ShortDurationInsuranceContractsAccidentYear2017Member thg:HomeownersMember 2019-12-31 0000944695 us-gaap:ShortDurationInsuranceContractsAccidentYear2018Member thg:HomeownersMember 2019-12-31 0000944695 thg:ShortDurationInsuranceContractsAccidentYear2019Member thg:HomeownersMember 2019-12-31 0000944695 thg:HomeownersMember 2019-12-31 0000944695 thg:ShortDurationInsuranceContractsAccidentYears2016To2019Member thg:HomeownersMember 2019-12-31 0000944695 thg:ShortDurationInsuranceContractsAccidentYears2015AndPriorMember thg:HomeownersMember 2019-12-31 0000944695 2019-01-02 2019-03-31 0000944695 2019-04-01 2019-06-30 0000944695 2019-07-01 2019-09-30 0000944695 2018-01-01 2018-03-31 0000944695 2018-04-01 2018-06-30 0000944695 2018-07-01 2018-09-30 0000944695 2019-12-05 2019-12-05 0000944695 2018-12-30 2018-12-30 0000944695 us-gaap:USStatesAndPoliticalSubdivisionsMember 2019-12-31 0000944695 us-gaap:ForeignGovernmentDebtSecuritiesMember 2019-12-31 0000944695 us-gaap:PublicUtilityBondsMember 2019-12-31 0000944695 us-gaap:AllOtherCorporateBondsMember 2019-12-31 0000944695 us-gaap:PublicUtilityEquitiesMember 2019-12-31 0000944695 us-gaap:BanksTrustAndInsuranceEquitiesMember 2019-12-31 0000944695 us-gaap:IndustrialMiscellaneousAndAllOthersMember 2019-12-31 0000944695 us-gaap:NonredeemablePreferredStockMember 2019-12-31 0000944695 us-gaap:EquitySecuritiesInvestmentSummaryMember 2019-12-31 0000944695 us-gaap:RealEstateInvestmentMember 2019-12-31 0000944695 us-gaap:OtherLongTermInvestmentsMember 2019-12-31 0000944695 us-gaap:ShortTermInvestmentsMember 2019-12-31 0000944695 srt:ParentCompanyMember 2019-01-01 2019-12-31 0000944695 srt:ParentCompanyMember 2018-01-01 2018-12-31 0000944695 srt:ParentCompanyMember 2017-01-01 2017-12-31 0000944695 srt:ParentCompanyMember thg:ChaucerMember 2019-01-01 2019-12-31 0000944695 srt:ParentCompanyMember thg:ChaucerMember 2018-01-01 2018-12-31 0000944695 srt:ParentCompanyMember 2019-12-31 0000944695 srt:ParentCompanyMember 2018-12-31 0000944695 srt:ParentCompanyMember thg:ChaucerMember 2019-12-31 0000944695 srt:ParentCompanyMember thg:ChaucerMember 2018-12-31 0000944695 srt:ParentCompanyMember thg:ChaucerMember 2019-01-01 2019-01-31 0000944695 srt:ParentCompanyMember 2017-12-31 0000944695 srt:ParentCompanyMember 2016-12-31 0000944695 thg:CommercialLinesSegmentMember us-gaap:OperatingSegmentsMember 2019-12-31 0000944695 thg:PersonalLinesSegmentMember us-gaap:OperatingSegmentsMember 2019-12-31 0000944695 us-gaap:AllOtherSegmentsMember us-gaap:OperatingSegmentsMember 2019-12-31 0000944695 thg:CommercialLinesSegmentMember us-gaap:OperatingSegmentsMember 2019-01-01 2019-12-31 0000944695 thg:PersonalLinesSegmentMember us-gaap:OperatingSegmentsMember 2019-01-01 2019-12-31 0000944695 us-gaap:AllOtherSegmentsMember us-gaap:OperatingSegmentsMember 2019-01-01 2019-12-31 0000944695 us-gaap:MaterialReconcilingItemsMember 2019-01-01 2019-12-31 0000944695 us-gaap:IntersegmentEliminationMember 2019-01-01 2019-12-31 0000944695 thg:CommercialLinesSegmentMember us-gaap:OperatingSegmentsMember 2018-12-31 0000944695 thg:PersonalLinesSegmentMember us-gaap:OperatingSegmentsMember 2018-12-31 0000944695 us-gaap:AllOtherSegmentsMember us-gaap:OperatingSegmentsMember 2018-12-31 0000944695 thg:CommercialLinesSegmentMember us-gaap:OperatingSegmentsMember 2018-01-01 2018-12-31 0000944695 thg:PersonalLinesSegmentMember us-gaap:OperatingSegmentsMember 2018-01-01 2018-12-31 0000944695 us-gaap:AllOtherSegmentsMember us-gaap:OperatingSegmentsMember 2018-01-01 2018-12-31 0000944695 us-gaap:MaterialReconcilingItemsMember 2018-01-01 2018-12-31 0000944695 us-gaap:IntersegmentEliminationMember 2018-01-01 2018-12-31 0000944695 thg:CommercialLinesSegmentMember us-gaap:OperatingSegmentsMember 2017-12-31 0000944695 thg:PersonalLinesSegmentMember us-gaap:OperatingSegmentsMember 2017-12-31 0000944695 us-gaap:AllOtherSegmentsMember us-gaap:OperatingSegmentsMember 2017-12-31 0000944695 us-gaap:IntersegmentEliminationMember 2017-12-31 0000944695 thg:CommercialLinesSegmentMember us-gaap:OperatingSegmentsMember 2017-01-01 2017-12-31 0000944695 thg:PersonalLinesSegmentMember us-gaap:OperatingSegmentsMember 2017-01-01 2017-12-31 0000944695 us-gaap:AllOtherSegmentsMember us-gaap:OperatingSegmentsMember 2017-01-01 2017-12-31 0000944695 us-gaap:MaterialReconcilingItemsMember 2017-01-01 2017-12-31 0000944695 us-gaap:IntersegmentEliminationMember 2017-01-01 2017-12-31 0000944695 us-gaap:AllowanceForCreditLossMember 2018-12-31 0000944695 us-gaap:AllowanceForReinsuranceRecoverableMember 2018-12-31 0000944695 us-gaap:AllowanceForCreditLossMember 2019-01-01 2019-12-31 0000944695 us-gaap:AllowanceForCreditLossMember 2019-12-31 0000944695 us-gaap:AllowanceForReinsuranceRecoverableMember 2019-12-31 0000944695 us-gaap:AllowanceForCreditLossMember 2017-12-31 0000944695 us-gaap:AllowanceForReinsuranceRecoverableMember 2017-12-31 0000944695 us-gaap:AllowanceForCreditLossMember 2018-01-01 2018-12-31 0000944695 us-gaap:AllowanceForReinsuranceRecoverableMember 2018-01-01 2018-12-31 0000944695 us-gaap:AllowanceForCreditLossMember 2016-12-31 0000944695 us-gaap:AllowanceForReinsuranceRecoverableMember 2016-12-31 0000944695 us-gaap:AllowanceForCreditLossMember 2017-01-01 2017-12-31 0000944695 us-gaap:AllowanceForReinsuranceRecoverableMember 2017-01-01 2017-12-31 0000944695 thg:ChaucerMember us-gaap:AllowanceForReinsuranceRecoverableMember 2018-01-01 2018-12-31 0000944695 thg:ChaucerMember us-gaap:AllowanceForReinsuranceRecoverableMember 2016-12-31 0000944695 thg:ChaucerMember us-gaap:AllowanceForReinsuranceRecoverableMember 2017-12-31 0000944695 srt:ConsolidatedPropertyAndCasualtyInsuranceEntityMember 2019-12-31 0000944695 srt:ConsolidatedPropertyAndCasualtyInsuranceEntityMember 2018-12-31 0000944695 srt:ConsolidatedPropertyAndCasualtyInsuranceEntityMember 2017-12-31 0000944695 srt:ConsolidatedPropertyAndCasualtyInsuranceEntityMember 2019-01-01 2019-12-31 0000944695 srt:ConsolidatedPropertyAndCasualtyInsuranceEntityMember 2018-01-01 2018-12-31 0000944695 srt:ConsolidatedPropertyAndCasualtyInsuranceEntityMember 2017-01-01 2017-12-31

☒	ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934

☐	TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934

Large accelerated filer	☒	Accelerated filer	☐
Non-accelerated filer	☐	Smaller reporting company	☐
		Emerging growth company	☐

Item		Description		Page
Part I
1		Business		3
1A.		Risk Factors		16
1B.		Unresolved Staff Comments		32
2		Properties		32
3		Legal Proceedings		32
4		Mine Safety Disclosures		32
Part II
5		Market for Registrant's Common Equity, Related Stockholder Matters and Issuer Purchases of Equity Securities		33
6		Selected Financial Data		34
7		Management's Discussion and Analysis of Financial Condition and Results of Operations		35
7A.		Quantitative and Qualitative Disclosures About Market Risk		70
8		Financial Statements and Supplementary Data		71
9		Changes in and Disagreements With Accountants on Accounting and Financial Disclosure		124
9A.		Controls and Procedures		124
9B.		Other Information		124
Part III
10		Directors, Executive Officers and Corporate Governance		125
11		Executive Compensation		127
12		Security Ownership of Certain Beneficial Owners and Management and Related Stockholder Matters		128
13		Certain Relationships and Related Transactions, and Director Independence		128
14		Principal Accounting Fees and Services		128
Part IV
15		Exhibits, Financial Statement Schedules		129
		Exhibits Index		130
16		Form 10-K Summary		133
		Signatures		134