Annual Report (10-k)

false2021FY0000230557http://fasb.org/us-gaap/2021-01-31#OtherAssetshttp://fasb.org/us-gaap/2021-01-31#OtherAssetshttp://fasb.org/us-gaap/2021-01-31#OtherLiabilitieshttp://fasb.org/us-gaap/2021-01-31#OtherLiabilitieshttp://fasb.org/us-gaap/2021-01-31#PropertyPlantAndEquipmentNethttp://fasb.org/us-gaap/2021-01-31#PropertyPlantAndEquipmentNethttp://fasb.org/us-gaap/2021-01-31#LongTermDebthttp://fasb.org/us-gaap/2021-01-31#LongTermDebt00002305572021-01-012021-12-310000230557us-gaap:CommonStockMember2021-01-012021-12-310000230557us-gaap:NoncumulativePreferredStockMember2021-01-012021-12-3100002305572021-06-30iso4217:USD00002305572022-01-31xbrli:shares00002305572021-12-3100002305572020-12-310000230557us-gaap:PreferredStockMember2021-12-31iso4217:USDxbrli:shares0000230557us-gaap:PreferredStockMember2020-12-310000230557us-gaap:SeriesBMember2021-12-310000230557us-gaap:CommonStockMember2021-12-310000230557us-gaap:CommonStockMember2020-12-310000230557us-gaap:TreasuryStockCommonMember2021-12-310000230557us-gaap:TreasuryStockCommonMember2020-12-3100002305572020-01-012020-12-3100002305572019-01-012019-12-310000230557us-gaap:PreferredStockMember2020-12-310000230557us-gaap:PreferredStockMember2019-12-310000230557us-gaap:PreferredStockMember2018-12-310000230557us-gaap:PreferredStockMember2021-01-012021-12-310000230557us-gaap:PreferredStockMember2020-01-012020-12-310000230557us-gaap:PreferredStockMember2019-01-012019-12-310000230557us-gaap:PreferredStockMember2021-12-310000230557us-gaap:CommonStockMember2020-12-310000230557us-gaap:CommonStockMember2019-12-310000230557us-gaap:CommonStockMember2018-12-310000230557us-gaap:CommonStockMember2021-01-012021-12-310000230557us-gaap:CommonStockMember2020-01-012020-12-310000230557us-gaap:CommonStockMember2019-01-012019-12-310000230557us-gaap:CommonStockMember2021-12-310000230557us-gaap:AdditionalPaidInCapitalMember2020-12-310000230557us-gaap:AdditionalPaidInCapitalMember2019-12-310000230557us-gaap:AdditionalPaidInCapitalMember2018-12-310000230557us-gaap:AdditionalPaidInCapitalMember2021-01-012021-12-310000230557us-gaap:AdditionalPaidInCapitalMember2020-01-012020-12-310000230557us-gaap:AdditionalPaidInCapitalMember2019-01-012019-12-310000230557us-gaap:AdditionalPaidInCapitalMember2021-12-310000230557us-gaap:RetainedEarningsMember2020-12-310000230557us-gaap:RetainedEarningsMember2019-12-310000230557us-gaap:RetainedEarningsMember2018-12-310000230557us-gaap:RetainedEarningsMemberus-gaap:AccountingStandardsUpdate201602Member2020-12-310000230557us-gaap:RetainedEarningsMemberus-gaap:AccountingStandardsUpdate201602Member2019-12-310000230557us-gaap:RetainedEarningsMemberus-gaap:AccountingStandardsUpdate201602Member2018-12-310000230557us-gaap:RetainedEarningsMemberus-gaap:AccountingStandardsUpdate201613Member2020-12-310000230557us-gaap:RetainedEarningsMemberus-gaap:AccountingStandardsUpdate201613Member2019-12-310000230557us-gaap:RetainedEarningsMemberus-gaap:AccountingStandardsUpdate201613Member2018-12-310000230557us-gaap:RetainedEarningsMembersrt:CumulativeEffectPeriodOfAdoptionAdjustmentMember2020-12-310000230557us-gaap:RetainedEarningsMembersrt:CumulativeEffectPeriodOfAdoptionAdjustmentMember2019-12-310000230557us-gaap:RetainedEarningsMembersrt:CumulativeEffectPeriodOfAdoptionAdjustmentMember2018-12-310000230557us-gaap:RetainedEarningsMember2021-01-012021-12-310000230557us-gaap:RetainedEarningsMember2020-01-012020-12-310000230557us-gaap:RetainedEarningsMember2019-01-012019-12-310000230557us-gaap:RetainedEarningsMember2021-12-310000230557us-gaap:AccumulatedOtherComprehensiveIncomeMember2020-12-310000230557us-gaap:AccumulatedOtherComprehensiveIncomeMember2019-12-310000230557us-gaap:AccumulatedOtherComprehensiveIncomeMember2018-12-310000230557us-gaap:AccumulatedOtherComprehensiveIncomeMember2021-01-012021-12-310000230557us-gaap:AccumulatedOtherComprehensiveIncomeMember2020-01-012020-12-310000230557us-gaap:AccumulatedOtherComprehensiveIncomeMember2019-01-012019-12-310000230557us-gaap:AccumulatedOtherComprehensiveIncomeMember2021-12-310000230557us-gaap:TreasuryStockMember2020-12-310000230557us-gaap:TreasuryStockMember2019-12-310000230557us-gaap:TreasuryStockMember2018-12-310000230557us-gaap:TreasuryStockMember2021-01-012021-12-310000230557us-gaap:TreasuryStockMember2020-01-012020-12-310000230557us-gaap:TreasuryStockMember2019-01-012019-12-310000230557us-gaap:TreasuryStockMember2021-12-3100002305572019-12-3100002305572018-12-310000230557us-gaap:ComputerEquipmentMember2021-01-012021-12-310000230557us-gaap:ComputerSoftwareIntangibleAssetMembersrt:MinimumMember2021-01-012021-12-310000230557us-gaap:ComputerSoftwareIntangibleAssetMembersrt:MaximumMember2021-01-012021-12-310000230557us-gaap:OtherIntangibleAssetsMembersrt:MinimumMember2021-01-012021-12-310000230557us-gaap:OtherIntangibleAssetsMembersrt:MaximumMember2021-01-012021-12-310000230557us-gaap:SoftwareDevelopmentMember2021-01-012021-12-310000230557us-gaap:FurnitureAndFixturesMember2021-01-012021-12-310000230557us-gaap:BuildingAndBuildingImprovementsMembersrt:MinimumMember2021-01-012021-12-310000230557srt:MaximumMemberus-gaap:BuildingAndBuildingImprovementsMember2021-01-012021-12-3100002305572021-01-012021-09-30xbrli:pure00002305572020-01-012020-09-3000002305572019-01-012019-09-300000230557us-gaap:AvailableforsaleSecuritiesMember2021-01-012021-12-310000230557us-gaap:AvailableforsaleSecuritiesMember2020-01-012020-12-310000230557us-gaap:AvailableforsaleSecuritiesMember2019-01-012019-12-310000230557us-gaap:HeldtomaturitySecuritiesMember2021-01-012021-12-310000230557us-gaap:HeldtomaturitySecuritiesMember2020-01-012020-12-310000230557us-gaap:HeldtomaturitySecuritiesMember2019-01-012019-12-310000230557us-gaap:EquitySecuritiesMember2021-01-012021-12-310000230557us-gaap:EquitySecuritiesMember2020-01-012020-12-310000230557us-gaap:EquitySecuritiesMember2019-01-012019-12-310000230557us-gaap:AvailableforsaleSecuritiesMember2021-12-310000230557us-gaap:AvailableforsaleSecuritiesMember2020-12-310000230557us-gaap:AvailableforsaleSecuritiesMember2019-12-310000230557us-gaap:HeldtomaturitySecuritiesMember2021-12-310000230557us-gaap:HeldtomaturitySecuritiesMember2020-12-310000230557us-gaap:HeldtomaturitySecuritiesMember2019-12-310000230557us-gaap:ShortTermInvestmentsMember2021-12-310000230557us-gaap:ShortTermInvestmentsMember2020-12-310000230557us-gaap:ShortTermInvestmentsMember2019-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:USTreasuryAndGovernmentMember2021-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:ForeignGovernmentDebtSecuritiesMember2021-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:USStatesAndPoliticalSubdivisionsMember2021-12-310000230557us-gaap:CorporateDebtSecuritiesMemberus-gaap:AvailableforsaleSecuritiesMember2021-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:AssetBackedSecuritiesMember2021-12-310000230557us-gaap:ResidentialMortgageBackedSecuritiesMemberus-gaap:AvailableforsaleSecuritiesMember2021-12-310000230557us-gaap:CommercialMortgageBackedSecuritiesMemberus-gaap:AvailableforsaleSecuritiesMember2021-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:USTreasuryAndGovernmentMember2020-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:ForeignGovernmentDebtSecuritiesMember2020-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:USStatesAndPoliticalSubdivisionsMember2020-12-310000230557us-gaap:CorporateDebtSecuritiesMemberus-gaap:AvailableforsaleSecuritiesMember2020-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:AssetBackedSecuritiesMember2020-12-310000230557us-gaap:ResidentialMortgageBackedSecuritiesMemberus-gaap:AvailableforsaleSecuritiesMember2020-12-310000230557us-gaap:CommercialMortgageBackedSecuritiesMemberus-gaap:AvailableforsaleSecuritiesMember2020-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:ForeignGovernmentDebtSecuritiesMember2021-01-012021-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:USStatesAndPoliticalSubdivisionsMember2021-01-012021-12-310000230557us-gaap:CorporateDebtSecuritiesMemberus-gaap:AvailableforsaleSecuritiesMember2021-01-012021-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:AssetBackedSecuritiesMember2021-01-012021-12-310000230557us-gaap:ResidentialMortgageBackedSecuritiesMemberus-gaap:AvailableforsaleSecuritiesMember2021-01-012021-12-310000230557us-gaap:CommercialMortgageBackedSecuritiesMemberus-gaap:AvailableforsaleSecuritiesMember2021-01-012021-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:ForeignGovernmentDebtSecuritiesMember2019-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:ForeignGovernmentDebtSecuritiesMember2020-01-012020-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:USStatesAndPoliticalSubdivisionsMember2019-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:USStatesAndPoliticalSubdivisionsMember2020-01-012020-12-310000230557us-gaap:CorporateDebtSecuritiesMemberus-gaap:AvailableforsaleSecuritiesMember2019-12-310000230557us-gaap:CorporateDebtSecuritiesMemberus-gaap:AvailableforsaleSecuritiesMember2020-01-012020-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:AssetBackedSecuritiesMember2019-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:AssetBackedSecuritiesMember2020-01-012020-12-310000230557us-gaap:ResidentialMortgageBackedSecuritiesMemberus-gaap:AvailableforsaleSecuritiesMember2019-12-310000230557us-gaap:ResidentialMortgageBackedSecuritiesMemberus-gaap:AvailableforsaleSecuritiesMember2020-01-012020-12-310000230557us-gaap:CommercialMortgageBackedSecuritiesMemberus-gaap:AvailableforsaleSecuritiesMember2019-12-310000230557us-gaap:CommercialMortgageBackedSecuritiesMemberus-gaap:AvailableforsaleSecuritiesMember2020-01-012020-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:USTreasuryAndGovernmentMember2021-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:ForeignGovernmentDebtSecuritiesMember2021-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:USStatesAndPoliticalSubdivisionsMember2021-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:CorporateDebtSecuritiesMember2021-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:AssetBackedSecuritiesMember2021-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:ResidentialMortgageBackedSecuritiesMember2021-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:CommercialMortgageBackedSecuritiesMember2021-12-310000230557us-gaap:AvailableforsaleSecuritiesMember2021-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:USTreasuryAndGovernmentMember2020-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:ForeignGovernmentDebtSecuritiesMember2020-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:USStatesAndPoliticalSubdivisionsMember2020-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:CorporateDebtSecuritiesMember2020-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:AssetBackedSecuritiesMember2020-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:ResidentialMortgageBackedSecuritiesMember2020-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:CommercialMortgageBackedSecuritiesMember2020-12-310000230557us-gaap:AvailableforsaleSecuritiesMember2020-12-310000230557us-gaap:PrivateEquityFundsMember2021-12-310000230557us-gaap:PrivateEquityFundsMember2020-12-310000230557sigi:PrivateCreditFundsMember2021-12-310000230557sigi:PrivateCreditFundsMember2020-12-310000230557sigi:RealAssetsMember2021-12-310000230557sigi:RealAssetsMember2020-12-310000230557srt:PartnershipInterestMember2021-12-310000230557srt:PartnershipInterestMember2020-12-310000230557sigi:OtherSecuritiesMember2021-12-310000230557sigi:OtherSecuritiesMember2020-12-310000230557us-gaap:OtherInvestmentsMember2021-12-310000230557us-gaap:OtherInvestmentsMember2020-12-310000230557sigi:InvestmentsAccountedForUnderTheEquityMethodMember2021-09-300000230557sigi:InvestmentsAccountedForUnderTheEquityMethodMember2020-09-300000230557us-gaap:EquityMethodInvestmentNonconsolidatedInvesteeOrGroupOfInvesteesMember2021-09-300000230557us-gaap:EquityMethodInvestmentNonconsolidatedInvesteeOrGroupOfInvesteesMember2020-09-300000230557us-gaap:EquityMethodInvestmentNonconsolidatedInvesteeOtherMember2021-09-300000230557us-gaap:EquityMethodInvestmentNonconsolidatedInvesteeOtherMember2020-09-300000230557sigi:InvestmentsAccountedForUnderTheEquityMethodMember2020-10-012021-09-300000230557sigi:InvestmentsAccountedForUnderTheEquityMethodMember2019-10-012020-09-300000230557sigi:InvestmentsAccountedForUnderTheEquityMethodMember2018-10-012019-09-300000230557srt:PartnershipInterestMember2020-10-012021-09-300000230557srt:PartnershipInterestMember2019-10-012020-09-300000230557srt:PartnershipInterestMember2018-10-012019-09-300000230557us-gaap:USTreasuryAndGovernmentMembersrt:FederalHomeLoanBankOfIndianapolisMember2021-12-310000230557us-gaap:USTreasuryAndGovernmentMembersrt:FederalHomeLoanBankOfNewYorkMember2021-12-310000230557us-gaap:USTreasuryAndGovernmentMember2021-12-310000230557us-gaap:USStatesAndPoliticalSubdivisionsMembersrt:FederalHomeLoanBankOfIndianapolisMember2021-12-310000230557us-gaap:USStatesAndPoliticalSubdivisionsMember2021-12-310000230557srt:FederalHomeLoanBankOfIndianapolisMemberus-gaap:ResidentialMortgageBackedSecuritiesMember2021-12-310000230557srt:FederalHomeLoanBankOfNewYorkMemberus-gaap:ResidentialMortgageBackedSecuritiesMember2021-12-310000230557us-gaap:ResidentialMortgageBackedSecuritiesMember2021-12-310000230557us-gaap:CommercialMortgageBackedSecuritiesMembersrt:FederalHomeLoanBankOfIndianapolisMember2021-12-310000230557us-gaap:CommercialMortgageBackedSecuritiesMembersrt:FederalHomeLoanBankOfNewYorkMember2021-12-310000230557us-gaap:CommercialMortgageBackedSecuritiesMember2021-12-310000230557us-gaap:AvailableforsaleSecuritiesMembersrt:FederalHomeLoanBankOfIndianapolisMember2021-12-310000230557us-gaap:AvailableforsaleSecuritiesMembersrt:FederalHomeLoanBankOfNewYorkMember2021-12-310000230557us-gaap:FixedMaturitiesMember2021-01-012021-12-310000230557us-gaap:FixedMaturitiesMember2020-01-012020-12-310000230557us-gaap:FixedMaturitiesMember2019-01-012019-12-310000230557sigi:CommercialMortgageLoansMember2021-01-012021-12-310000230557sigi:CommercialMortgageLoansMember2020-01-012020-12-310000230557sigi:CommercialMortgageLoansMember2019-01-012019-12-310000230557us-gaap:ShortTermInvestmentsMember2021-01-012021-12-310000230557us-gaap:ShortTermInvestmentsMember2020-01-012020-12-310000230557us-gaap:ShortTermInvestmentsMember2019-01-012019-12-310000230557us-gaap:OtherInvestmentsMember2021-01-012021-12-310000230557us-gaap:OtherInvestmentsMember2020-01-012020-12-310000230557us-gaap:OtherInvestmentsMember2019-01-012019-12-310000230557sigi:InvestmentExpensesMember2021-01-012021-12-310000230557sigi:InvestmentExpensesMember2020-01-012020-12-310000230557sigi:InvestmentExpensesMember2019-01-012019-12-310000230557us-gaap:AccumulatedOtherThanTemporaryImpairmentMember2019-12-310000230557us-gaap:HeldtomaturitySecuritiesMemberus-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember2019-12-310000230557us-gaap:AvailableforsaleSecuritiesMember2019-12-310000230557us-gaap:InvestmentsMember2019-12-310000230557us-gaap:AccumulatedDefinedBenefitPlansAdjustmentMember2019-12-310000230557us-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember2020-01-012020-12-310000230557us-gaap:HeldtomaturitySecuritiesMemberus-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember2020-01-012020-12-310000230557us-gaap:AvailableforsaleSecuritiesMember2020-01-012020-12-310000230557us-gaap:InvestmentsMember2020-01-012020-12-310000230557us-gaap:AccumulatedDefinedBenefitPlansAdjustmentMember2020-01-012020-12-310000230557us-gaap:AccumulatedOtherThanTemporaryImpairmentMember2020-01-012020-12-310000230557us-gaap:AccumulatedOtherThanTemporaryImpairmentMember2020-12-310000230557us-gaap:HeldtomaturitySecuritiesMemberus-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember2020-12-310000230557us-gaap:AvailableforsaleSecuritiesMember2020-12-310000230557us-gaap:InvestmentsMember2020-12-310000230557us-gaap:AccumulatedDefinedBenefitPlansAdjustmentMember2020-12-310000230557us-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember2021-01-012021-12-310000230557us-gaap:HeldtomaturitySecuritiesMemberus-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember2021-01-012021-12-310000230557us-gaap:AvailableforsaleSecuritiesMember2021-01-012021-12-310000230557us-gaap:InvestmentsMember2021-01-012021-12-310000230557us-gaap:AccumulatedDefinedBenefitPlansAdjustmentMember2021-01-012021-12-310000230557us-gaap:AccumulatedOtherThanTemporaryImpairmentMember2021-01-012021-12-310000230557us-gaap:AccumulatedOtherThanTemporaryImpairmentMember2021-12-310000230557us-gaap:HeldtomaturitySecuritiesMemberus-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember2021-12-310000230557us-gaap:AvailableforsaleSecuritiesMember2021-12-310000230557us-gaap:InvestmentsMember2021-12-310000230557us-gaap:AccumulatedDefinedBenefitPlansAdjustmentMember2021-12-310000230557us-gaap:HeldtomaturitySecuritiesMemberus-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember2021-01-012021-12-310000230557us-gaap:HeldtomaturitySecuritiesMemberus-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember2020-01-012020-12-310000230557us-gaap:AociGainLossDebtSecuritiesAvailableForSaleWithoutAllowanceForCreditLossParentMemberus-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember2021-01-012021-12-310000230557us-gaap:AociGainLossDebtSecuritiesAvailableForSaleWithoutAllowanceForCreditLossParentMemberus-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember2020-01-012020-12-310000230557us-gaap:AociGainLossDebtSecuritiesAvailableForSaleWithAllowanceForCreditLossParentMemberus-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember2021-01-012021-12-310000230557us-gaap:AociGainLossDebtSecuritiesAvailableForSaleWithAllowanceForCreditLossParentMemberus-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember2020-01-012020-12-310000230557us-gaap:PensionPlansDefinedBenefitMemberus-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember2021-01-012021-12-310000230557us-gaap:PensionPlansDefinedBenefitMemberus-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember2020-01-012020-12-310000230557us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember2021-01-012021-12-310000230557us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember2020-01-012020-12-310000230557sigi:SevenPointTwentyFivePercentSeniorNotesMemberus-gaap:CarryingReportedAmountFairValueDisclosureMember2021-12-310000230557sigi:SevenPointTwentyFivePercentSeniorNotesMemberus-gaap:EstimateOfFairValueFairValueDisclosureMember2021-12-310000230557sigi:SevenPointTwentyFivePercentSeniorNotesMemberus-gaap:CarryingReportedAmountFairValueDisclosureMember2020-12-310000230557sigi:SevenPointTwentyFivePercentSeniorNotesMemberus-gaap:EstimateOfFairValueFairValueDisclosureMember2020-12-310000230557sigi:SixPointSeventyPercentSeniorNotesMemberus-gaap:CarryingReportedAmountFairValueDisclosureMember2021-12-310000230557us-gaap:EstimateOfFairValueFairValueDisclosureMembersigi:SixPointSeventyPercentSeniorNotesMember2021-12-310000230557sigi:SixPointSeventyPercentSeniorNotesMemberus-gaap:CarryingReportedAmountFairValueDisclosureMember2020-12-310000230557us-gaap:EstimateOfFairValueFairValueDisclosureMembersigi:SixPointSeventyPercentSeniorNotesMember2020-12-310000230557sigi:FivePointThreeHundredSeventyFiveMemberus-gaap:CarryingReportedAmountFairValueDisclosureMember2021-12-310000230557sigi:FivePointThreeHundredSeventyFiveMemberus-gaap:EstimateOfFairValueFairValueDisclosureMember2021-12-310000230557sigi:FivePointThreeHundredSeventyFiveMemberus-gaap:CarryingReportedAmountFairValueDisclosureMember2020-12-310000230557sigi:FivePointThreeHundredSeventyFiveMemberus-gaap:EstimateOfFairValueFairValueDisclosureMember2020-12-310000230557sigi:OnePointSixtyOnePercentBorrowingFromFederalHomeLoanBankDueJuly212021Memberus-gaap:CarryingReportedAmountFairValueDisclosureMember2021-12-310000230557sigi:OnePointSixtyOnePercentBorrowingFromFederalHomeLoanBankDueJuly212021Memberus-gaap:EstimateOfFairValueFairValueDisclosureMember2021-12-310000230557sigi:OnePointSixtyOnePercentBorrowingFromFederalHomeLoanBankDueJuly212021Memberus-gaap:CarryingReportedAmountFairValueDisclosureMember2020-12-310000230557sigi:OnePointSixtyOnePercentBorrowingFromFederalHomeLoanBankDueJuly212021Memberus-gaap:EstimateOfFairValueFairValueDisclosureMember2020-12-310000230557sigi:OnePointFiftySixPercentBorrowingFromFederalHomeLoanBankDueAugust162021Memberus-gaap:CarryingReportedAmountFairValueDisclosureMember2021-12-310000230557sigi:OnePointFiftySixPercentBorrowingFromFederalHomeLoanBankDueAugust162021Memberus-gaap:EstimateOfFairValueFairValueDisclosureMember2021-12-310000230557sigi:OnePointFiftySixPercentBorrowingFromFederalHomeLoanBankDueAugust162021Memberus-gaap:CarryingReportedAmountFairValueDisclosureMember2020-12-310000230557sigi:OnePointFiftySixPercentBorrowingFromFederalHomeLoanBankDueAugust162021Memberus-gaap:EstimateOfFairValueFairValueDisclosureMember2020-12-310000230557sigi:ThreePointZeroThreePercentBorrowingFromFederalHomeLoanBankDueDecember162026Memberus-gaap:CarryingReportedAmountFairValueDisclosureMember2021-12-310000230557sigi:ThreePointZeroThreePercentBorrowingFromFederalHomeLoanBankDueDecember162026Memberus-gaap:EstimateOfFairValueFairValueDisclosureMember2021-12-310000230557sigi:ThreePointZeroThreePercentBorrowingFromFederalHomeLoanBankDueDecember162026Memberus-gaap:CarryingReportedAmountFairValueDisclosureMember2020-12-310000230557sigi:ThreePointZeroThreePercentBorrowingFromFederalHomeLoanBankDueDecember162026Memberus-gaap:EstimateOfFairValueFairValueDisclosureMember2020-12-310000230557sigi:LongTermDebtExcludingIssuanceCostsMemberus-gaap:CarryingReportedAmountFairValueDisclosureMember2021-12-310000230557us-gaap:EstimateOfFairValueFairValueDisclosureMember2021-12-310000230557sigi:LongTermDebtExcludingIssuanceCostsMemberus-gaap:CarryingReportedAmountFairValueDisclosureMember2020-12-310000230557us-gaap:EstimateOfFairValueFairValueDisclosureMember2020-12-310000230557sigi:UnamortizedDebtIssuanceCostsMemberus-gaap:CarryingReportedAmountFairValueDisclosureMember2021-12-310000230557sigi:UnamortizedDebtIssuanceCostsMemberus-gaap:CarryingReportedAmountFairValueDisclosureMember2020-12-310000230557sigi:FinanceleaseobligationMemberus-gaap:CarryingReportedAmountFairValueDisclosureMember2021-12-310000230557sigi:FinanceleaseobligationMemberus-gaap:CarryingReportedAmountFairValueDisclosureMember2020-12-310000230557us-gaap:CarryingReportedAmountFairValueDisclosureMember2021-12-310000230557us-gaap:CarryingReportedAmountFairValueDisclosureMember2020-12-310000230557us-gaap:USTreasuryAndGovernmentMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:USTreasuryAndGovernmentMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:USTreasuryAndGovernmentMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:USTreasuryAndGovernmentMemberus-gaap:FairValueInputsLevel3Memberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:ForeignGovernmentDebtSecuritiesMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:ForeignGovernmentDebtSecuritiesMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:ForeignGovernmentDebtSecuritiesMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:ForeignGovernmentDebtSecuritiesMemberus-gaap:FairValueInputsLevel3Memberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:USStatesAndPoliticalSubdivisionsMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:USStatesAndPoliticalSubdivisionsMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:USStatesAndPoliticalSubdivisionsMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:FairValueInputsLevel3Memberus-gaap:USStatesAndPoliticalSubdivisionsMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:CorporateDebtSecuritiesMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:CorporateDebtSecuritiesMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:CorporateDebtSecuritiesMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:CorporateDebtSecuritiesMemberus-gaap:FairValueInputsLevel3Memberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:FairValueMeasurementsRecurringMemberus-gaap:AssetBackedSecuritiesMember2021-12-310000230557us-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMemberus-gaap:AssetBackedSecuritiesMember2021-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:FairValueMeasurementsRecurringMemberus-gaap:AssetBackedSecuritiesMember2021-12-310000230557us-gaap:FairValueInputsLevel3Memberus-gaap:FairValueMeasurementsRecurringMemberus-gaap:AssetBackedSecuritiesMember2021-12-310000230557us-gaap:FairValueMeasurementsRecurringMemberus-gaap:ResidentialMortgageBackedSecuritiesMember2021-12-310000230557us-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMemberus-gaap:ResidentialMortgageBackedSecuritiesMember2021-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:FairValueMeasurementsRecurringMemberus-gaap:ResidentialMortgageBackedSecuritiesMember2021-12-310000230557us-gaap:FairValueInputsLevel3Memberus-gaap:FairValueMeasurementsRecurringMemberus-gaap:ResidentialMortgageBackedSecuritiesMember2021-12-310000230557us-gaap:CommercialMortgageBackedSecuritiesMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:CommercialMortgageBackedSecuritiesMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:CommercialMortgageBackedSecuritiesMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:FairValueInputsLevel3Memberus-gaap:CommercialMortgageBackedSecuritiesMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:FairValueInputsLevel3Memberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:CommonStockMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:CommonStockMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:CommonStockMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:CommonStockMemberus-gaap:FairValueInputsLevel3Memberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:NonredeemablePreferredStockMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:NonredeemablePreferredStockMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:NonredeemablePreferredStockMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:NonredeemablePreferredStockMemberus-gaap:FairValueInputsLevel3Memberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:USTreasuryAndGovernmentMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:USTreasuryAndGovernmentMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:USTreasuryAndGovernmentMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:USTreasuryAndGovernmentMemberus-gaap:FairValueInputsLevel3Memberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:ForeignGovernmentDebtSecuritiesMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:ForeignGovernmentDebtSecuritiesMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:ForeignGovernmentDebtSecuritiesMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:ForeignGovernmentDebtSecuritiesMemberus-gaap:FairValueInputsLevel3Memberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:USStatesAndPoliticalSubdivisionsMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:USStatesAndPoliticalSubdivisionsMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:USStatesAndPoliticalSubdivisionsMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:FairValueInputsLevel3Memberus-gaap:USStatesAndPoliticalSubdivisionsMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:CorporateDebtSecuritiesMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:CorporateDebtSecuritiesMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:CorporateDebtSecuritiesMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:CorporateDebtSecuritiesMemberus-gaap:FairValueInputsLevel3Memberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:FairValueMeasurementsRecurringMemberus-gaap:AssetBackedSecuritiesMember2020-12-310000230557us-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMemberus-gaap:AssetBackedSecuritiesMember2020-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:FairValueMeasurementsRecurringMemberus-gaap:AssetBackedSecuritiesMember2020-12-310000230557us-gaap:FairValueInputsLevel3Memberus-gaap:FairValueMeasurementsRecurringMemberus-gaap:AssetBackedSecuritiesMember2020-12-310000230557us-gaap:CommercialMortgageBackedSecuritiesMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:CommercialMortgageBackedSecuritiesMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:CommercialMortgageBackedSecuritiesMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:FairValueInputsLevel3Memberus-gaap:CommercialMortgageBackedSecuritiesMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:FairValueMeasurementsRecurringMemberus-gaap:ResidentialMortgageBackedSecuritiesMember2020-12-310000230557us-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMemberus-gaap:ResidentialMortgageBackedSecuritiesMember2020-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:FairValueMeasurementsRecurringMemberus-gaap:ResidentialMortgageBackedSecuritiesMember2020-12-310000230557us-gaap:FairValueInputsLevel3Memberus-gaap:FairValueMeasurementsRecurringMemberus-gaap:ResidentialMortgageBackedSecuritiesMember2020-12-310000230557us-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:FairValueInputsLevel3Memberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:CommonStockMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:CommonStockMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:CommonStockMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:CommonStockMemberus-gaap:FairValueInputsLevel3Memberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:NonredeemablePreferredStockMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:NonredeemablePreferredStockMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:NonredeemablePreferredStockMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:NonredeemablePreferredStockMemberus-gaap:FairValueInputsLevel3Memberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:USStatesAndPoliticalSubdivisionsMember2020-12-310000230557us-gaap:CorporateDebtSecuritiesMemberus-gaap:AvailableforsaleSecuritiesMember2020-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:AssetBackedSecuritiesMember2020-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:ResidentialMortgageBackedSecuritiesMember2020-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:CommercialMortgageBackedSecuritiesMember2020-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:InvestmentsMember2020-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:USStatesAndPoliticalSubdivisionsMember2021-01-012021-12-310000230557us-gaap:CorporateDebtSecuritiesMemberus-gaap:AvailableforsaleSecuritiesMember2021-01-012021-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:AssetBackedSecuritiesMember2021-01-012021-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:ResidentialMortgageBackedSecuritiesMember2021-01-012021-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:CommercialMortgageBackedSecuritiesMember2021-01-012021-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:InvestmentsMember2021-01-012021-12-310000230557us-gaap:GainLossOnInvestmentsMember1us-gaap:AvailableforsaleSecuritiesMemberus-gaap:USStatesAndPoliticalSubdivisionsMember2021-01-012021-12-310000230557sigi:NetInvestmentIncomeEarnedMemberus-gaap:CorporateDebtSecuritiesMemberus-gaap:AvailableforsaleSecuritiesMember2021-01-012021-12-310000230557sigi:NetInvestmentIncomeEarnedMemberus-gaap:AvailableforsaleSecuritiesMemberus-gaap:AssetBackedSecuritiesMember2021-01-012021-12-310000230557sigi:NetInvestmentIncomeEarnedMemberus-gaap:AvailableforsaleSecuritiesMemberus-gaap:ResidentialMortgageBackedSecuritiesMember2021-01-012021-12-310000230557sigi:NetInvestmentIncomeEarnedMemberus-gaap:AvailableforsaleSecuritiesMemberus-gaap:CommercialMortgageBackedSecuritiesMember2021-01-012021-12-310000230557sigi:NetInvestmentIncomeEarnedMemberus-gaap:AvailableforsaleSecuritiesMember2021-01-012021-12-310000230557sigi:NetInvestmentIncomeEarnedMemberus-gaap:AvailableforsaleSecuritiesMemberus-gaap:USStatesAndPoliticalSubdivisionsMember2021-01-012021-12-310000230557sigi:NetInvestmentIncomeEarnedMemberus-gaap:AvailableforsaleSecuritiesMemberus-gaap:InvestmentsMember2021-01-012021-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:USStatesAndPoliticalSubdivisionsMember2021-12-310000230557us-gaap:CorporateDebtSecuritiesMemberus-gaap:AvailableforsaleSecuritiesMember2021-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:AssetBackedSecuritiesMember2021-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:ResidentialMortgageBackedSecuritiesMember2021-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:CommercialMortgageBackedSecuritiesMember2021-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:InvestmentsMember2021-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:USStatesAndPoliticalSubdivisionsMember2019-12-310000230557us-gaap:CorporateDebtSecuritiesMemberus-gaap:AvailableforsaleSecuritiesMember2019-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:AssetBackedSecuritiesMember2019-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:InvestmentsMember2019-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:USStatesAndPoliticalSubdivisionsMember2020-01-012020-12-310000230557us-gaap:CorporateDebtSecuritiesMemberus-gaap:AvailableforsaleSecuritiesMember2020-01-012020-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:AssetBackedSecuritiesMember2020-01-012020-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:InvestmentsMember2020-01-012020-12-310000230557us-gaap:GainLossOnInvestmentsMember1us-gaap:AvailableforsaleSecuritiesMemberus-gaap:USStatesAndPoliticalSubdivisionsMember2020-01-012020-12-310000230557us-gaap:GainLossOnInvestmentsMember1us-gaap:CorporateDebtSecuritiesMemberus-gaap:AvailableforsaleSecuritiesMember2020-01-012020-12-310000230557us-gaap:GainLossOnInvestmentsMember1us-gaap:AvailableforsaleSecuritiesMemberus-gaap:AssetBackedSecuritiesMember2020-01-012020-12-310000230557us-gaap:GainLossOnInvestmentsMember1us-gaap:AvailableforsaleSecuritiesMemberus-gaap:InvestmentsMember2020-01-012020-12-310000230557sigi:NetInvestmentIncomeEarnedMemberus-gaap:AvailableforsaleSecuritiesMemberus-gaap:USStatesAndPoliticalSubdivisionsMember2020-01-012020-12-310000230557sigi:NetInvestmentIncomeEarnedMemberus-gaap:CorporateDebtSecuritiesMemberus-gaap:AvailableforsaleSecuritiesMember2020-01-012020-12-310000230557sigi:NetInvestmentIncomeEarnedMemberus-gaap:AvailableforsaleSecuritiesMemberus-gaap:AssetBackedSecuritiesMember2020-01-012020-12-310000230557sigi:NetInvestmentIncomeEarnedMemberus-gaap:AvailableforsaleSecuritiesMemberus-gaap:InvestmentsMember2020-01-012020-12-310000230557us-gaap:CorporateDebtSecuritiesMemberus-gaap:AvailableforsaleSecuritiesMemberus-gaap:ValuationTechniqueDiscountedCashFlowMember2021-12-310000230557us-gaap:CorporateDebtSecuritiesMemberus-gaap:MeasurementInputCreditSpreadMemberus-gaap:ValuationTechniqueDiscountedCashFlowMembersrt:MinimumMember2021-12-310000230557us-gaap:CorporateDebtSecuritiesMembersrt:MaximumMemberus-gaap:MeasurementInputCreditSpreadMemberus-gaap:ValuationTechniqueDiscountedCashFlowMember2021-12-310000230557srt:WeightedAverageMemberus-gaap:CorporateDebtSecuritiesMemberus-gaap:MeasurementInputCreditSpreadMemberus-gaap:ValuationTechniqueDiscountedCashFlowMember2021-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:ValuationTechniqueDiscountedCashFlowMemberus-gaap:AssetBackedSecuritiesMember2021-12-310000230557us-gaap:MeasurementInputCreditSpreadMemberus-gaap:ValuationTechniqueDiscountedCashFlowMembersrt:MinimumMemberus-gaap:AssetBackedSecuritiesMember2021-12-310000230557srt:MaximumMemberus-gaap:MeasurementInputCreditSpreadMemberus-gaap:ValuationTechniqueDiscountedCashFlowMemberus-gaap:AssetBackedSecuritiesMember2021-12-310000230557srt:WeightedAverageMemberus-gaap:MeasurementInputCreditSpreadMemberus-gaap:ValuationTechniqueDiscountedCashFlowMemberus-gaap:AssetBackedSecuritiesMember2021-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:ValuationTechniqueDiscountedCashFlowMemberus-gaap:InvestmentsMember2021-12-310000230557us-gaap:CorporateDebtSecuritiesMemberus-gaap:AvailableforsaleSecuritiesMemberus-gaap:ValuationTechniqueDiscountedCashFlowMember2020-12-310000230557us-gaap:CorporateDebtSecuritiesMemberus-gaap:MeasurementInputCreditSpreadMemberus-gaap:ValuationTechniqueDiscountedCashFlowMembersrt:MinimumMember2020-12-310000230557us-gaap:CorporateDebtSecuritiesMembersrt:MaximumMemberus-gaap:MeasurementInputCreditSpreadMemberus-gaap:ValuationTechniqueDiscountedCashFlowMember2020-12-310000230557srt:WeightedAverageMemberus-gaap:CorporateDebtSecuritiesMemberus-gaap:MeasurementInputCreditSpreadMemberus-gaap:ValuationTechniqueDiscountedCashFlowMember2020-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:ValuationTechniqueDiscountedCashFlowMemberus-gaap:AssetBackedSecuritiesMember2020-12-310000230557us-gaap:MeasurementInputCreditSpreadMemberus-gaap:ValuationTechniqueDiscountedCashFlowMembersrt:MinimumMemberus-gaap:AssetBackedSecuritiesMember2020-12-310000230557srt:MaximumMemberus-gaap:MeasurementInputCreditSpreadMemberus-gaap:ValuationTechniqueDiscountedCashFlowMemberus-gaap:AssetBackedSecuritiesMember2020-12-310000230557srt:WeightedAverageMemberus-gaap:MeasurementInputCreditSpreadMemberus-gaap:ValuationTechniqueDiscountedCashFlowMemberus-gaap:AssetBackedSecuritiesMember2020-12-310000230557us-gaap:AvailableforsaleSecuritiesMemberus-gaap:ValuationTechniqueDiscountedCashFlowMemberus-gaap:InvestmentsMember2020-12-310000230557us-gaap:USStatesAndPoliticalSubdivisionsMemberus-gaap:FairValueInputsLevel1Member2021-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:USStatesAndPoliticalSubdivisionsMember2021-12-310000230557us-gaap:USStatesAndPoliticalSubdivisionsMemberus-gaap:FairValueInputsLevel3Member2021-12-310000230557us-gaap:CorporateDebtSecuritiesMember2021-12-310000230557us-gaap:CorporateDebtSecuritiesMemberus-gaap:FairValueInputsLevel1Member2021-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:CorporateDebtSecuritiesMember2021-12-310000230557us-gaap:CorporateDebtSecuritiesMemberus-gaap:FairValueInputsLevel3Member2021-12-310000230557us-gaap:FairValueInputsLevel1Member2021-12-310000230557us-gaap:FairValueInputsLevel2Member2021-12-310000230557us-gaap:FairValueInputsLevel3Member2021-12-310000230557sigi:SevenPointTwentyFivePercentSeniorNotesMember2021-12-310000230557sigi:SevenPointTwentyFivePercentSeniorNotesMemberus-gaap:FairValueInputsLevel1Member2021-12-310000230557us-gaap:FairValueInputsLevel2Membersigi:SevenPointTwentyFivePercentSeniorNotesMember2021-12-310000230557us-gaap:FairValueInputsLevel3Membersigi:SevenPointTwentyFivePercentSeniorNotesMember2021-12-310000230557sigi:SixPointSeventyPercentSeniorNotesMember2021-12-310000230557us-gaap:FairValueInputsLevel1Membersigi:SixPointSeventyPercentSeniorNotesMember2021-12-310000230557us-gaap:FairValueInputsLevel2Membersigi:SixPointSeventyPercentSeniorNotesMember2021-12-310000230557us-gaap:FairValueInputsLevel3Membersigi:SixPointSeventyPercentSeniorNotesMember2021-12-310000230557sigi:FivePointThreeHundredSeventyFiveMember2021-12-310000230557sigi:FivePointThreeHundredSeventyFiveMemberus-gaap:FairValueInputsLevel1Member2021-12-310000230557sigi:FivePointThreeHundredSeventyFiveMemberus-gaap:FairValueInputsLevel2Member2021-12-310000230557sigi:FivePointThreeHundredSeventyFiveMemberus-gaap:FairValueInputsLevel3Member2021-12-310000230557sigi:ThreePointZeroThreePercentBorrowingFromFederalHomeLoanBankDueDecember162026Member2021-12-310000230557sigi:ThreePointZeroThreePercentBorrowingFromFederalHomeLoanBankDueDecember162026Memberus-gaap:FairValueInputsLevel1Member2021-12-310000230557us-gaap:FairValueInputsLevel2Membersigi:ThreePointZeroThreePercentBorrowingFromFederalHomeLoanBankDueDecember162026Member2021-12-310000230557sigi:ThreePointZeroThreePercentBorrowingFromFederalHomeLoanBankDueDecember162026Memberus-gaap:FairValueInputsLevel3Member2021-12-310000230557us-gaap:LongTermDebtMember2021-12-310000230557us-gaap:USStatesAndPoliticalSubdivisionsMember2020-12-310000230557us-gaap:USStatesAndPoliticalSubdivisionsMemberus-gaap:FairValueInputsLevel1Member2020-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:USStatesAndPoliticalSubdivisionsMember2020-12-310000230557us-gaap:USStatesAndPoliticalSubdivisionsMemberus-gaap:FairValueInputsLevel3Member2020-12-310000230557us-gaap:CorporateDebtSecuritiesMember2020-12-310000230557us-gaap:CorporateDebtSecuritiesMemberus-gaap:FairValueInputsLevel1Member2020-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:CorporateDebtSecuritiesMember2020-12-310000230557us-gaap:CorporateDebtSecuritiesMemberus-gaap:FairValueInputsLevel3Member2020-12-310000230557us-gaap:FairValueInputsLevel1Member2020-12-310000230557us-gaap:FairValueInputsLevel2Member2020-12-310000230557us-gaap:FairValueInputsLevel3Member2020-12-310000230557sigi:SevenPointTwentyFivePercentSeniorNotesMember2020-12-310000230557sigi:SevenPointTwentyFivePercentSeniorNotesMemberus-gaap:FairValueInputsLevel1Member2020-12-310000230557us-gaap:FairValueInputsLevel2Membersigi:SevenPointTwentyFivePercentSeniorNotesMember2020-12-310000230557us-gaap:FairValueInputsLevel3Membersigi:SevenPointTwentyFivePercentSeniorNotesMember2020-12-310000230557sigi:SixPointSeventyPercentSeniorNotesMember2020-12-310000230557us-gaap:FairValueInputsLevel1Membersigi:SixPointSeventyPercentSeniorNotesMember2020-12-310000230557us-gaap:FairValueInputsLevel2Membersigi:SixPointSeventyPercentSeniorNotesMember2020-12-310000230557us-gaap:FairValueInputsLevel3Membersigi:SixPointSeventyPercentSeniorNotesMember2020-12-310000230557sigi:FivePointThreeHundredSeventyFiveMember2020-12-310000230557sigi:FivePointThreeHundredSeventyFiveMemberus-gaap:FairValueInputsLevel1Member2020-12-310000230557sigi:FivePointThreeHundredSeventyFiveMemberus-gaap:FairValueInputsLevel2Member2020-12-310000230557sigi:FivePointThreeHundredSeventyFiveMemberus-gaap:FairValueInputsLevel3Member2020-12-310000230557sigi:OnePointSixtyOnePercentBorrowingFromFederalHomeLoanBankDueJuly212021Member2020-12-310000230557sigi:OnePointSixtyOnePercentBorrowingFromFederalHomeLoanBankDueJuly212021Memberus-gaap:FairValueInputsLevel1Member2020-12-310000230557us-gaap:FairValueInputsLevel2Membersigi:OnePointSixtyOnePercentBorrowingFromFederalHomeLoanBankDueJuly212021Member2020-12-310000230557us-gaap:FairValueInputsLevel3Membersigi:OnePointSixtyOnePercentBorrowingFromFederalHomeLoanBankDueJuly212021Member2020-12-310000230557sigi:OnePointFiftySixPercentBorrowingFromFederalHomeLoanBankDueAugust162021Member2020-12-310000230557sigi:OnePointFiftySixPercentBorrowingFromFederalHomeLoanBankDueAugust162021Memberus-gaap:FairValueInputsLevel1Member2020-12-310000230557us-gaap:FairValueInputsLevel2Membersigi:OnePointFiftySixPercentBorrowingFromFederalHomeLoanBankDueAugust162021Member2020-12-310000230557sigi:OnePointFiftySixPercentBorrowingFromFederalHomeLoanBankDueAugust162021Memberus-gaap:FairValueInputsLevel3Member2020-12-310000230557sigi:ThreePointZeroThreePercentBorrowingFromFederalHomeLoanBankDueDecember162026Member2020-12-310000230557sigi:ThreePointZeroThreePercentBorrowingFromFederalHomeLoanBankDueDecember162026Memberus-gaap:FairValueInputsLevel1Member2020-12-310000230557us-gaap:FairValueInputsLevel2Membersigi:ThreePointZeroThreePercentBorrowingFromFederalHomeLoanBankDueDecember162026Member2020-12-310000230557sigi:ThreePointZeroThreePercentBorrowingFromFederalHomeLoanBankDueDecember162026Memberus-gaap:FairValueInputsLevel3Member2020-12-310000230557us-gaap:LongTermDebtMember2020-12-310000230557us-gaap:PremiumsReceivableMemberus-gaap:RetainedEarningsMemberus-gaap:AccountingStandardsUpdate201613Member2020-12-310000230557us-gaap:PremiumsReceivableMemberus-gaap:RetainedEarningsMemberus-gaap:AccountingStandardsUpdate201613Member2019-12-310000230557us-gaap:PremiumsReceivableMember2020-12-310000230557us-gaap:PremiumsReceivableMember2019-12-310000230557us-gaap:PremiumsReceivableMember2021-01-012021-12-310000230557us-gaap:PremiumsReceivableMember2020-01-012020-12-310000230557us-gaap:PremiumsReceivableMember2021-12-310000230557srt:AMBestAPlusPlusRatingMemberus-gaap:FinancialAssetNotPastDueMember2021-12-310000230557srt:AMBestAPlusPlusRatingMemberus-gaap:FinancialAssetPastDueMember2021-12-310000230557srt:AMBestAPlusPlusRatingMember2021-12-310000230557srt:AMBestAPlusRatingMemberus-gaap:FinancialAssetNotPastDueMember2021-12-310000230557srt:AMBestAPlusRatingMemberus-gaap:FinancialAssetPastDueMember2021-12-310000230557srt:AMBestAPlusRatingMember2021-12-310000230557srt:AMBestARatingMemberus-gaap:FinancialAssetNotPastDueMember2021-12-310000230557us-gaap:FinancialAssetPastDueMembersrt:AMBestARatingMember2021-12-310000230557srt:AMBestARatingMember2021-12-310000230557srt:AMBestAMinusRatingMemberus-gaap:FinancialAssetNotPastDueMember2021-12-310000230557srt:AMBestAMinusRatingMemberus-gaap:FinancialAssetPastDueMember2021-12-310000230557srt:AMBestAMinusRatingMember2021-12-310000230557srt:AMBestBPlusPlusRatingMemberus-gaap:FinancialAssetNotPastDueMember2021-12-310000230557srt:AMBestBPlusPlusRatingMemberus-gaap:FinancialAssetPastDueMember2021-12-310000230557srt:AMBestBPlusPlusRatingMember2021-12-310000230557srt:AMBestBPlusRatingMemberus-gaap:FinancialAssetNotPastDueMember2021-12-310000230557srt:AMBestBPlusRatingMemberus-gaap:FinancialAssetPastDueMember2021-12-310000230557srt:AMBestBPlusRatingMember2021-12-310000230557us-gaap:FinancialAssetNotPastDueMembersigi:AMBestRatedReinsurerMember2021-12-310000230557us-gaap:FinancialAssetPastDueMembersigi:AMBestRatedReinsurerMember2021-12-310000230557sigi:AMBestRatedReinsurerMember2021-12-310000230557us-gaap:FinancialAssetNotPastDueMembersigi:TotalfederalandstatepoolsMember2021-12-310000230557us-gaap:FinancialAssetPastDueMembersigi:TotalfederalandstatepoolsMember2021-12-310000230557sigi:TotalfederalandstatepoolsMember2021-12-310000230557sigi:OtherFederalAndStatePoolsMemberus-gaap:FinancialAssetNotPastDueMember2021-12-310000230557sigi:OtherFederalAndStatePoolsMemberus-gaap:FinancialAssetPastDueMember2021-12-310000230557sigi:OtherFederalAndStatePoolsMember2021-12-310000230557us-gaap:FinancialAssetNotPastDueMembersigi:TotalNonRatedReinsurerMember2021-12-310000230557us-gaap:FinancialAssetPastDueMembersigi:TotalNonRatedReinsurerMember2021-12-310000230557sigi:TotalNonRatedReinsurerMember2021-12-310000230557us-gaap:FinancialAssetNotPastDueMembersigi:ReinsuranceRecoverableforPaidandUnpaidClaimsandClaimsAdjustmentsGrossMember2021-12-310000230557us-gaap:FinancialAssetPastDueMembersigi:ReinsuranceRecoverableforPaidandUnpaidClaimsandClaimsAdjustmentsGrossMember2021-12-310000230557srt:AMBestAPlusPlusRatingMemberus-gaap:FinancialAssetNotPastDueMember2020-12-310000230557srt:AMBestAPlusPlusRatingMemberus-gaap:FinancialAssetPastDueMember2020-12-310000230557srt:AMBestAPlusPlusRatingMember2020-12-310000230557srt:AMBestAPlusRatingMemberus-gaap:FinancialAssetNotPastDueMember2020-12-310000230557srt:AMBestAPlusRatingMemberus-gaap:FinancialAssetPastDueMember2020-12-310000230557srt:AMBestAPlusRatingMember2020-12-310000230557srt:AMBestARatingMemberus-gaap:FinancialAssetNotPastDueMember2020-12-310000230557us-gaap:FinancialAssetPastDueMembersrt:AMBestARatingMember2020-12-310000230557srt:AMBestARatingMember2020-12-310000230557srt:AMBestAMinusRatingMemberus-gaap:FinancialAssetNotPastDueMember2020-12-310000230557srt:AMBestAMinusRatingMemberus-gaap:FinancialAssetPastDueMember2020-12-310000230557srt:AMBestAMinusRatingMember2020-12-310000230557srt:AMBestBPlusPlusRatingMemberus-gaap:FinancialAssetNotPastDueMember2020-12-310000230557srt:AMBestBPlusPlusRatingMemberus-gaap:FinancialAssetPastDueMember2020-12-310000230557srt:AMBestBPlusPlusRatingMember2020-12-310000230557srt:AMBestBPlusRatingMemberus-gaap:FinancialAssetNotPastDueMember2020-12-310000230557srt:AMBestBPlusRatingMemberus-gaap:FinancialAssetPastDueMember2020-12-310000230557srt:AMBestBPlusRatingMember2020-12-310000230557us-gaap:FinancialAssetNotPastDueMembersigi:AMBestRatedReinsurerMember2020-12-310000230557us-gaap:FinancialAssetPastDueMembersigi:AMBestRatedReinsurerMember2020-12-310000230557sigi:AMBestRatedReinsurerMember2020-12-310000230557us-gaap:FinancialAssetNotPastDueMembersigi:TotalfederalandstatepoolsMember2020-12-310000230557us-gaap:FinancialAssetPastDueMembersigi:TotalfederalandstatepoolsMember2020-12-310000230557sigi:TotalfederalandstatepoolsMember2020-12-310000230557sigi:OtherFederalAndStatePoolsMemberus-gaap:FinancialAssetNotPastDueMember2020-12-310000230557sigi:OtherFederalAndStatePoolsMemberus-gaap:FinancialAssetPastDueMember2020-12-310000230557sigi:OtherFederalAndStatePoolsMember2020-12-310000230557us-gaap:FinancialAssetNotPastDueMembersigi:TotalNonRatedReinsurerMember2020-12-310000230557us-gaap:FinancialAssetPastDueMembersigi:TotalNonRatedReinsurerMember2020-12-310000230557sigi:TotalNonRatedReinsurerMember2020-12-310000230557us-gaap:FinancialAssetNotPastDueMembersigi:ReinsuranceRecoverableforPaidandUnpaidClaimsandClaimsAdjustmentsGrossMember2020-12-310000230557us-gaap:FinancialAssetPastDueMembersigi:ReinsuranceRecoverableforPaidandUnpaidClaimsandClaimsAdjustmentsGrossMember2020-12-310000230557us-gaap:RetainedEarningsMembersigi:ReinsuranceRecoverableforPaidandUnpaidClaimsandClaimsAdjustmentsGrossMemberus-gaap:AccountingStandardsUpdate201613Member2020-12-310000230557us-gaap:RetainedEarningsMembersigi:ReinsuranceRecoverableforPaidandUnpaidClaimsandClaimsAdjustmentsGrossMemberus-gaap:AccountingStandardsUpdate201613Member2019-12-310000230557us-gaap:AccountingStandardsUpdate201613Member2020-12-310000230557us-gaap:AccountingStandardsUpdate201613Member2019-12-310000230557sigi:NationalFloodInsuranceProgramMember2021-12-310000230557sigi:NationalFloodInsuranceProgramMember2020-12-310000230557sigi:NjUnsatisfiedClaimJudgmentFundMember2021-12-310000230557sigi:NjUnsatisfiedClaimJudgmentFundMember2020-12-310000230557sigi:OtherFederalAndStatePoolsMember2021-12-310000230557sigi:OtherFederalAndStatePoolsMember2020-12-310000230557sigi:TotalFederalAndStatePoolsMember2021-12-310000230557sigi:TotalFederalAndStatePoolsMember2020-12-310000230557sigi:RemainingUnsecuredReinsuranceMember2021-12-310000230557sigi:RemainingUnsecuredReinsuranceMember2020-12-310000230557sigi:MunichReGroupMember2021-12-310000230557sigi:MunichReGroupMember2020-12-310000230557sigi:HanoverRuckversicherungsAgMember2021-12-310000230557sigi:HanoverRuckversicherungsAgMember2020-12-310000230557sigi:AxisReinsuranceCompanyMember2021-12-310000230557sigi:AxisReinsuranceCompanyMember2020-12-310000230557sigi:SwissReGroupMember2021-12-310000230557sigi:SwissReGroupMember2020-12-310000230557sigi:TransatlanticReinsuranceCompanyMember2021-12-310000230557sigi:TransatlanticReinsuranceCompanyMember2020-12-310000230557sigi:AllOtherReinsurersMember2021-12-310000230557sigi:AllOtherReinsurersMember2020-12-310000230557sigi:RemainingUnsecuredReinsuranceNetofCollateralMember2021-12-310000230557sigi:RemainingUnsecuredReinsuranceNetofCollateralMember2020-12-310000230557sigi:ReturnauditandmidtermendorsementpremiumMembersigi:COVID19Member2021-01-012021-12-310000230557sigi:CommercialPersonalAutomobileMembersigi:COVID19Member2021-01-012021-12-310000230557sigi:NationalFloodInsuranceProgramMember2021-01-012021-12-310000230557sigi:NationalFloodInsuranceProgramMember2020-01-012020-12-310000230557sigi:NationalFloodInsuranceProgramMember2019-01-012019-12-310000230557us-gaap:RetainedEarningsMemberus-gaap:AccountingStandardsUpdate201613Member2019-01-012019-12-310000230557us-gaap:RetainedEarningsMemberus-gaap:AccountingStandardsUpdate201613Member2021-12-310000230557sigi:ChangeinHistoricalClaimRateExperienceMember2021-01-012021-12-310000230557sigi:ChangeinHistoricalClaimRateExperienceMember2020-01-012020-12-310000230557sigi:ChangeinHistoricalClaimRateExperienceMember2019-01-012019-12-310000230557us-gaap:GeneralLiabilityMembersigi:ChangeinHistoricalClaimRateExperienceMember2021-01-012021-12-310000230557us-gaap:GeneralLiabilityMembersigi:ChangeinHistoricalClaimRateExperienceMember2020-01-012020-12-310000230557us-gaap:GeneralLiabilityMembersigi:ChangeinHistoricalClaimRateExperienceMember2019-01-012019-12-310000230557sigi:CommercialAutomobileMembersigi:ChangeinHistoricalClaimRateExperienceMember2021-01-012021-12-310000230557sigi:CommercialAutomobileMembersigi:ChangeinHistoricalClaimRateExperienceMember2020-01-012020-12-310000230557sigi:CommercialAutomobileMembersigi:ChangeinHistoricalClaimRateExperienceMember2019-01-012019-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:ChangeinHistoricalClaimRateExperienceMember2021-01-012021-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:ChangeinHistoricalClaimRateExperienceMember2020-01-012020-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:ChangeinHistoricalClaimRateExperienceMember2019-01-012019-12-310000230557sigi:ChangeinHistoricalClaimRateExperienceMembersigi:BusinessOwnersPoliciesMember2021-01-012021-12-310000230557sigi:ChangeinHistoricalClaimRateExperienceMembersigi:BusinessOwnersPoliciesMember2020-01-012020-12-310000230557sigi:ChangeinHistoricalClaimRateExperienceMembersigi:BusinessOwnersPoliciesMember2019-01-012019-12-310000230557sigi:ChangeinHistoricalClaimRateExperienceMemberus-gaap:PropertyAndCasualtyCommercialInsuranceProductLineMember2021-01-012021-12-310000230557sigi:ChangeinHistoricalClaimRateExperienceMemberus-gaap:PropertyAndCasualtyCommercialInsuranceProductLineMember2020-01-012020-12-310000230557sigi:ChangeinHistoricalClaimRateExperienceMemberus-gaap:PropertyAndCasualtyCommercialInsuranceProductLineMember2019-01-012019-12-310000230557sigi:HomeownersMembersigi:ChangeinHistoricalClaimRateExperienceMember2021-01-012021-12-310000230557sigi:HomeownersMembersigi:ChangeinHistoricalClaimRateExperienceMember2020-01-012020-12-310000230557sigi:HomeownersMembersigi:ChangeinHistoricalClaimRateExperienceMember2019-01-012019-12-310000230557sigi:PersonalAutomobileMembersigi:ChangeinHistoricalClaimRateExperienceMember2021-01-012021-12-310000230557sigi:PersonalAutomobileMembersigi:ChangeinHistoricalClaimRateExperienceMember2020-01-012020-12-310000230557sigi:PersonalAutomobileMembersigi:ChangeinHistoricalClaimRateExperienceMember2019-01-012019-12-310000230557sigi:ESCasualtyLinesMembersigi:ChangeinHistoricalClaimRateExperienceMember2021-01-012021-12-310000230557sigi:ESCasualtyLinesMembersigi:ChangeinHistoricalClaimRateExperienceMember2020-01-012020-12-310000230557sigi:ESCasualtyLinesMembersigi:ChangeinHistoricalClaimRateExperienceMember2019-01-012019-12-310000230557sigi:ESPropertyLinesMembersigi:ChangeinHistoricalClaimRateExperienceMember2021-01-012021-12-310000230557sigi:ESPropertyLinesMembersigi:ChangeinHistoricalClaimRateExperienceMember2020-01-012020-12-310000230557sigi:ESPropertyLinesMembersigi:ChangeinHistoricalClaimRateExperienceMember2019-01-012019-12-310000230557sigi:ChangeinHistoricalClaimRateExperienceMemberus-gaap:OtherInsuranceProductLineMember2021-01-012021-12-310000230557sigi:ChangeinHistoricalClaimRateExperienceMemberus-gaap:OtherInsuranceProductLineMember2020-01-012020-12-310000230557sigi:ChangeinHistoricalClaimRateExperienceMemberus-gaap:OtherInsuranceProductLineMember2019-01-012019-12-310000230557sigi:ChangeinHistoricalClaimRateExperienceMembersigi:CasualtyInsuranceProductLineMember2021-01-012021-12-310000230557sigi:ChangeinHistoricalClaimRateExperienceMemberus-gaap:PropertyInsuranceProductLineMember2021-01-012021-12-310000230557sigi:ChangeinHistoricalClaimRateExperienceMembersigi:CasualtyInsuranceProductLineMembersigi:CommercialAutomobileMember2021-01-012021-12-310000230557sigi:ChangeinHistoricalClaimRateExperienceMemberus-gaap:PropertyInsuranceProductLineMembersigi:CommercialAutomobileMember2021-01-012021-12-310000230557sigi:ChangeinHistoricalClaimRateExperienceMembersigi:CasualtyInsuranceProductLineMember2020-01-012020-12-310000230557sigi:ChangeinHistoricalClaimRateExperienceMemberus-gaap:PropertyInsuranceProductLineMember2020-01-012020-12-310000230557sigi:ChangeinHistoricalClaimRateExperienceMembersigi:CasualtyInsuranceProductLineMembersigi:CommercialAutomobileMember2020-01-012020-12-310000230557sigi:ChangeinHistoricalClaimRateExperienceMemberus-gaap:PropertyInsuranceProductLineMembersigi:CommercialAutomobileMember2020-01-012020-12-310000230557sigi:ChangeinHistoricalClaimRateExperienceMembersigi:CasualtyInsuranceProductLineMember2019-01-012019-12-310000230557sigi:ChangeinHistoricalClaimRateExperienceMemberus-gaap:PropertyInsuranceProductLineMember2019-01-012019-12-310000230557sigi:AsbestosClaimMember2021-12-310000230557sigi:LandfillSitesRelatedClaimsMember2021-12-310000230557sigi:LeakingUndergroundStorageTanksMember2021-12-310000230557us-gaap:AsbestosIssueMember2020-12-310000230557us-gaap:AsbestosIssueMember2019-12-310000230557us-gaap:AsbestosIssueMember2018-12-310000230557us-gaap:AsbestosIssueMember2021-01-012021-12-310000230557us-gaap:AsbestosIssueMember2020-01-012020-12-310000230557us-gaap:AsbestosIssueMember2019-01-012019-12-310000230557us-gaap:AsbestosIssueMember2021-12-310000230557sigi:EnvironmentalclaimsMember2020-12-310000230557sigi:EnvironmentalclaimsMember2019-12-310000230557sigi:EnvironmentalclaimsMember2018-12-310000230557sigi:EnvironmentalclaimsMember2021-01-012021-12-310000230557sigi:EnvironmentalclaimsMember2020-01-012020-12-310000230557sigi:EnvironmentalclaimsMember2019-01-012019-12-310000230557sigi:EnvironmentalclaimsMember2021-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2012-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2013-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2014-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2015-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2016-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2017-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2018-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2019-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2020-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2021-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2013-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2014-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2015-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2016-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2017-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2018-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2019-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2020-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2021-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2014-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2015-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2016-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2017-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2018-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2019-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2020-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2021-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2015-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2016-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2017-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2018-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2019-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2020-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2021-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2016-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2017-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2018-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2019-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2020-12-310000230557us-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2021-12-310000230557us-gaap:ShortDurationInsuranceContractsAccidentYear2017Member2017-12-310000230557us-gaap:ShortDurationInsuranceContractsAccidentYear2017Member2018-12-310000230557us-gaap:ShortDurationInsuranceContractsAccidentYear2017Member2019-12-310000230557us-gaap:ShortDurationInsuranceContractsAccidentYear2017Member2020-12-310000230557us-gaap:ShortDurationInsuranceContractsAccidentYear2017Member2021-12-310000230557us-gaap:ShortDurationInsuranceContractsAccidentYear2018Member2018-12-310000230557us-gaap:ShortDurationInsuranceContractsAccidentYear2018Member2019-12-310000230557us-gaap:ShortDurationInsuranceContractsAccidentYear2018Member2020-12-310000230557us-gaap:ShortDurationInsuranceContractsAccidentYear2018Member2021-12-310000230557us-gaap:ShortDurationInsuranceContractAccidentYear2019Member2019-12-310000230557us-gaap:ShortDurationInsuranceContractAccidentYear2019Member2020-12-310000230557us-gaap:ShortDurationInsuranceContractAccidentYear2019Member2021-12-310000230557us-gaap:ShortDurationInsuranceContractAccidentYear2020Member2020-12-310000230557us-gaap:ShortDurationInsuranceContractAccidentYear2020Member2021-12-310000230557us-gaap:ShortDurationInsuranceContractAccidentYear2021Member2021-12-310000230557sigi:ShortDurationInsuranceContractsAccidentYearsPriorTo2012Member2021-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2012-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2013-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2014-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2015-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2016-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2017-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2018-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2019-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2020-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2021-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2013-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2014-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2015-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2016-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2017-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2018-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2019-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2020-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2021-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2014-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2015-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2016-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2017-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2018-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2019-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2020-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2021-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2015-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2016-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2017-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2018-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2019-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2020-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2021-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2016-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2017-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2018-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2019-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2020-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2021-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Member2017-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Member2018-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Member2019-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Member2020-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Member2021-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2018Member2018-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2018Member2019-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2018Member2020-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2018Member2021-12-310000230557us-gaap:ShortDurationInsuranceContractAccidentYear2019Membersigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMember2019-12-310000230557us-gaap:ShortDurationInsuranceContractAccidentYear2019Membersigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMember2020-12-310000230557us-gaap:ShortDurationInsuranceContractAccidentYear2019Membersigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMember2021-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortDurationInsuranceContractAccidentYear2020Member2020-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortDurationInsuranceContractAccidentYear2020Member2021-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortDurationInsuranceContractAccidentYear2021Member2021-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMember2021-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:GeneralLiabilityMembersigi:ShortDurationInsuranceContractsAccidentYearsPriorTo2012Member2021-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2012-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2013-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2014-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2015-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2016-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2017-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2018-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2019-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2020-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2021-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2013-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2014-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2015-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2016-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2017-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2018-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2019-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2020-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2021-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2014-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2015-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2016-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2017-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2018-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2019-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2020-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2021-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2015-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2016-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2017-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2018-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2019-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2020-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2021-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2016-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2017-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2018-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2019-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2020-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2021-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Member2017-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Member2018-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Member2019-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Member2020-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Member2021-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2018Member2018-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2018Member2019-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2018Member2020-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2018Member2021-12-310000230557us-gaap:WorkersCompensationInsuranceMemberus-gaap:ShortDurationInsuranceContractAccidentYear2019Membersigi:StandardCommercialLinesMember2019-12-310000230557us-gaap:WorkersCompensationInsuranceMemberus-gaap:ShortDurationInsuranceContractAccidentYear2019Membersigi:StandardCommercialLinesMember2020-12-310000230557us-gaap:WorkersCompensationInsuranceMemberus-gaap:ShortDurationInsuranceContractAccidentYear2019Membersigi:StandardCommercialLinesMember2021-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractAccidentYear2020Member2020-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractAccidentYear2020Member2021-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractAccidentYear2021Member2021-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMember2021-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMembersigi:ShortDurationInsuranceContractsAccidentYearsPriorTo2012Member2021-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2012-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2013-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2014-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2015-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2016-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2017-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2018-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2019-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2020-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2021-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2013-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2014-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2015-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2016-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2017-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2018-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2019-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2020-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2021-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2014-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2015-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2016-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2017-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2018-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2019-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2020-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2021-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2015-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2016-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2017-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2018-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2019-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2020-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2021-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2016-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2017-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2018-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2019-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2020-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2021-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Member2017-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Member2018-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Member2019-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Member2020-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Member2021-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2018Member2018-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2018Member2019-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2018Member2020-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2018Member2021-12-310000230557us-gaap:ShortDurationInsuranceContractAccidentYear2019Membersigi:CommercialAutomobileMembersigi:StandardCommercialLinesMember2019-12-310000230557us-gaap:ShortDurationInsuranceContractAccidentYear2019Membersigi:CommercialAutomobileMembersigi:StandardCommercialLinesMember2020-12-310000230557us-gaap:ShortDurationInsuranceContractAccidentYear2019Membersigi:CommercialAutomobileMembersigi:StandardCommercialLinesMember2021-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractAccidentYear2020Member2020-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractAccidentYear2020Member2021-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractAccidentYear2021Member2021-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMember2021-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMembersigi:ShortDurationInsuranceContractsAccidentYearsPriorTo2012Member2021-12-310000230557sigi:StandardCommercialLinesMembersigi:BusinessOwnersPoliciesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2012-12-310000230557sigi:StandardCommercialLinesMembersigi:BusinessOwnersPoliciesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2013-12-310000230557sigi:StandardCommercialLinesMembersigi:BusinessOwnersPoliciesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2014-12-310000230557sigi:StandardCommercialLinesMembersigi:BusinessOwnersPoliciesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2015-12-310000230557sigi:StandardCommercialLinesMembersigi:BusinessOwnersPoliciesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2016-12-310000230557sigi:StandardCommercialLinesMembersigi:BusinessOwnersPoliciesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2017-12-310000230557sigi:StandardCommercialLinesMembersigi:BusinessOwnersPoliciesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2018-12-310000230557sigi:StandardCommercialLinesMembersigi:BusinessOwnersPoliciesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2019-12-310000230557sigi:StandardCommercialLinesMembersigi:BusinessOwnersPoliciesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2020-12-310000230557sigi:StandardCommercialLinesMembersigi:BusinessOwnersPoliciesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2021-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Membersigi:BusinessOwnersPoliciesMember2013-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Membersigi:BusinessOwnersPoliciesMember2014-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Membersigi:BusinessOwnersPoliciesMember2015-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Membersigi:BusinessOwnersPoliciesMember2016-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Membersigi:BusinessOwnersPoliciesMember2017-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Membersigi:BusinessOwnersPoliciesMember2018-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Membersigi:BusinessOwnersPoliciesMember2019-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Membersigi:BusinessOwnersPoliciesMember2020-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Membersigi:BusinessOwnersPoliciesMember2021-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Membersigi:BusinessOwnersPoliciesMember2014-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Membersigi:BusinessOwnersPoliciesMember2015-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Membersigi:BusinessOwnersPoliciesMember2016-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Membersigi:BusinessOwnersPoliciesMember2017-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Membersigi:BusinessOwnersPoliciesMember2018-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Membersigi:BusinessOwnersPoliciesMember2019-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Membersigi:BusinessOwnersPoliciesMember2020-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Membersigi:BusinessOwnersPoliciesMember2021-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Membersigi:BusinessOwnersPoliciesMember2015-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Membersigi:BusinessOwnersPoliciesMember2016-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Membersigi:BusinessOwnersPoliciesMember2017-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Membersigi:BusinessOwnersPoliciesMember2018-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Membersigi:BusinessOwnersPoliciesMember2019-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Membersigi:BusinessOwnersPoliciesMember2020-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Membersigi:BusinessOwnersPoliciesMember2021-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Membersigi:BusinessOwnersPoliciesMember2016-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Membersigi:BusinessOwnersPoliciesMember2017-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Membersigi:BusinessOwnersPoliciesMember2018-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Membersigi:BusinessOwnersPoliciesMember2019-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Membersigi:BusinessOwnersPoliciesMember2020-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Membersigi:BusinessOwnersPoliciesMember2021-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Membersigi:BusinessOwnersPoliciesMember2017-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Membersigi:BusinessOwnersPoliciesMember2018-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Membersigi:BusinessOwnersPoliciesMember2019-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Membersigi:BusinessOwnersPoliciesMember2020-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Membersigi:BusinessOwnersPoliciesMember2021-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2018Membersigi:BusinessOwnersPoliciesMember2018-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2018Membersigi:BusinessOwnersPoliciesMember2019-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2018Membersigi:BusinessOwnersPoliciesMember2020-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2018Membersigi:BusinessOwnersPoliciesMember2021-12-310000230557us-gaap:ShortDurationInsuranceContractAccidentYear2019Membersigi:StandardCommercialLinesMembersigi:BusinessOwnersPoliciesMember2019-12-310000230557us-gaap:ShortDurationInsuranceContractAccidentYear2019Membersigi:StandardCommercialLinesMembersigi:BusinessOwnersPoliciesMember2020-12-310000230557us-gaap:ShortDurationInsuranceContractAccidentYear2019Membersigi:StandardCommercialLinesMembersigi:BusinessOwnersPoliciesMember2021-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractAccidentYear2020Membersigi:BusinessOwnersPoliciesMember2020-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractAccidentYear2020Membersigi:BusinessOwnersPoliciesMember2021-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractAccidentYear2021Membersigi:BusinessOwnersPoliciesMember2021-12-310000230557sigi:StandardCommercialLinesMembersigi:BusinessOwnersPoliciesMember2021-12-310000230557sigi:StandardCommercialLinesMembersigi:ShortDurationInsuranceContractsAccidentYearsPriorTo2012Membersigi:BusinessOwnersPoliciesMember2021-12-310000230557sigi:StandardCommercialLinesMembersigi:CommercialPropertyMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2012-12-310000230557sigi:StandardCommercialLinesMembersigi:CommercialPropertyMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2013-12-310000230557sigi:StandardCommercialLinesMembersigi:CommercialPropertyMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2014-12-310000230557sigi:StandardCommercialLinesMembersigi:CommercialPropertyMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2015-12-310000230557sigi:StandardCommercialLinesMembersigi:CommercialPropertyMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2016-12-310000230557sigi:StandardCommercialLinesMembersigi:CommercialPropertyMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2017-12-310000230557sigi:StandardCommercialLinesMembersigi:CommercialPropertyMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2018-12-310000230557sigi:StandardCommercialLinesMembersigi:CommercialPropertyMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2019-12-310000230557sigi:StandardCommercialLinesMembersigi:CommercialPropertyMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2020-12-310000230557sigi:StandardCommercialLinesMembersigi:CommercialPropertyMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2021-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Membersigi:CommercialPropertyMember2013-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Membersigi:CommercialPropertyMember2014-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Membersigi:CommercialPropertyMember2015-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Membersigi:CommercialPropertyMember2016-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Membersigi:CommercialPropertyMember2017-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Membersigi:CommercialPropertyMember2018-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Membersigi:CommercialPropertyMember2019-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Membersigi:CommercialPropertyMember2020-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Membersigi:CommercialPropertyMember2021-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Membersigi:CommercialPropertyMember2014-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Membersigi:CommercialPropertyMember2015-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Membersigi:CommercialPropertyMember2016-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Membersigi:CommercialPropertyMember2017-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Membersigi:CommercialPropertyMember2018-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Membersigi:CommercialPropertyMember2019-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Membersigi:CommercialPropertyMember2020-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Membersigi:CommercialPropertyMember2021-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Membersigi:CommercialPropertyMember2015-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Membersigi:CommercialPropertyMember2016-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Membersigi:CommercialPropertyMember2017-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Membersigi:CommercialPropertyMember2018-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Membersigi:CommercialPropertyMember2019-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Membersigi:CommercialPropertyMember2020-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Membersigi:CommercialPropertyMember2021-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Membersigi:CommercialPropertyMember2016-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Membersigi:CommercialPropertyMember2017-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Membersigi:CommercialPropertyMember2018-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Membersigi:CommercialPropertyMember2019-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Membersigi:CommercialPropertyMember2020-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Membersigi:CommercialPropertyMember2021-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Membersigi:CommercialPropertyMember2017-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Membersigi:CommercialPropertyMember2018-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Membersigi:CommercialPropertyMember2019-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Membersigi:CommercialPropertyMember2020-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Membersigi:CommercialPropertyMember2021-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2018Membersigi:CommercialPropertyMember2018-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2018Membersigi:CommercialPropertyMember2019-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2018Membersigi:CommercialPropertyMember2020-12-310000230557sigi:StandardCommercialLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2018Membersigi:CommercialPropertyMember2021-12-310000230557us-gaap:ShortDurationInsuranceContractAccidentYear2019Membersigi:StandardCommercialLinesMembersigi:CommercialPropertyMember2019-12-310000230557us-gaap:ShortDurationInsuranceContractAccidentYear2019Membersigi:StandardCommercialLinesMembersigi:CommercialPropertyMember2020-12-310000230557us-gaap:ShortDurationInsuranceContractAccidentYear2019Membersigi:StandardCommercialLinesMembersigi:CommercialPropertyMember2021-12-310000230557sigi:StandardCommercialLinesMembersigi:CommercialPropertyMemberus-gaap:ShortDurationInsuranceContractAccidentYear2020Member2020-12-310000230557sigi:StandardCommercialLinesMembersigi:CommercialPropertyMemberus-gaap:ShortDurationInsuranceContractAccidentYear2020Member2021-12-310000230557sigi:StandardCommercialLinesMembersigi:CommercialPropertyMemberus-gaap:ShortDurationInsuranceContractAccidentYear2021Member2021-12-310000230557sigi:StandardCommercialLinesMembersigi:CommercialPropertyMember2021-12-310000230557sigi:StandardCommercialLinesMembersigi:ShortDurationInsuranceContractsAccidentYearsPriorTo2012Membersigi:CommercialPropertyMember2021-12-310000230557sigi:PersonalAutomobileMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2012-12-310000230557sigi:PersonalAutomobileMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2013-12-310000230557sigi:PersonalAutomobileMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2014-12-310000230557sigi:PersonalAutomobileMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2015-12-310000230557sigi:PersonalAutomobileMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2016-12-310000230557sigi:PersonalAutomobileMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2017-12-310000230557sigi:PersonalAutomobileMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2018-12-310000230557sigi:PersonalAutomobileMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2019-12-310000230557sigi:PersonalAutomobileMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2020-12-310000230557sigi:PersonalAutomobileMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2021-12-310000230557sigi:PersonalAutomobileMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Membersigi:StandardPersonalLinesMember2013-12-310000230557sigi:PersonalAutomobileMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Membersigi:StandardPersonalLinesMember2014-12-310000230557sigi:PersonalAutomobileMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Membersigi:StandardPersonalLinesMember2015-12-310000230557sigi:PersonalAutomobileMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Membersigi:StandardPersonalLinesMember2016-12-310000230557sigi:PersonalAutomobileMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Membersigi:StandardPersonalLinesMember2017-12-310000230557sigi:PersonalAutomobileMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Membersigi:StandardPersonalLinesMember2018-12-310000230557sigi:PersonalAutomobileMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Membersigi:StandardPersonalLinesMember2019-12-310000230557sigi:PersonalAutomobileMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Membersigi:StandardPersonalLinesMember2020-12-310000230557sigi:PersonalAutomobileMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Membersigi:StandardPersonalLinesMember2021-12-310000230557sigi:PersonalAutomobileMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Membersigi:StandardPersonalLinesMember2014-12-310000230557sigi:PersonalAutomobileMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Membersigi:StandardPersonalLinesMember2015-12-310000230557sigi:PersonalAutomobileMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Membersigi:StandardPersonalLinesMember2016-12-310000230557sigi:PersonalAutomobileMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Membersigi:StandardPersonalLinesMember2017-12-310000230557sigi:PersonalAutomobileMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Membersigi:StandardPersonalLinesMember2018-12-310000230557sigi:PersonalAutomobileMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Membersigi:StandardPersonalLinesMember2019-12-310000230557sigi:PersonalAutomobileMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Membersigi:StandardPersonalLinesMember2020-12-310000230557sigi:PersonalAutomobileMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Membersigi:StandardPersonalLinesMember2021-12-310000230557sigi:PersonalAutomobileMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2015-12-310000230557sigi:PersonalAutomobileMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2016-12-310000230557sigi:PersonalAutomobileMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2017-12-310000230557sigi:PersonalAutomobileMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2018-12-310000230557sigi:PersonalAutomobileMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2019-12-310000230557sigi:PersonalAutomobileMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2020-12-310000230557sigi:PersonalAutomobileMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2021-12-310000230557sigi:PersonalAutomobileMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2016-12-310000230557sigi:PersonalAutomobileMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2017-12-310000230557sigi:PersonalAutomobileMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2018-12-310000230557sigi:PersonalAutomobileMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2019-12-310000230557sigi:PersonalAutomobileMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2020-12-310000230557sigi:PersonalAutomobileMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2021-12-310000230557sigi:PersonalAutomobileMembersigi:StandardPersonalLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Member2017-12-310000230557sigi:PersonalAutomobileMembersigi:StandardPersonalLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Member2018-12-310000230557sigi:PersonalAutomobileMembersigi:StandardPersonalLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Member2019-12-310000230557sigi:PersonalAutomobileMembersigi:StandardPersonalLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Member2020-12-310000230557sigi:PersonalAutomobileMembersigi:StandardPersonalLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Member2021-12-310000230557sigi:PersonalAutomobileMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2018Membersigi:StandardPersonalLinesMember2018-12-310000230557sigi:PersonalAutomobileMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2018Membersigi:StandardPersonalLinesMember2019-12-310000230557sigi:PersonalAutomobileMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2018Membersigi:StandardPersonalLinesMember2020-12-310000230557sigi:PersonalAutomobileMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2018Membersigi:StandardPersonalLinesMember2021-12-310000230557us-gaap:ShortDurationInsuranceContractAccidentYear2019Membersigi:PersonalAutomobileMembersigi:StandardPersonalLinesMember2019-12-310000230557us-gaap:ShortDurationInsuranceContractAccidentYear2019Membersigi:PersonalAutomobileMembersigi:StandardPersonalLinesMember2020-12-310000230557us-gaap:ShortDurationInsuranceContractAccidentYear2019Membersigi:PersonalAutomobileMembersigi:StandardPersonalLinesMember2021-12-310000230557sigi:PersonalAutomobileMembersigi:StandardPersonalLinesMemberus-gaap:ShortDurationInsuranceContractAccidentYear2020Member2020-12-310000230557sigi:PersonalAutomobileMembersigi:StandardPersonalLinesMemberus-gaap:ShortDurationInsuranceContractAccidentYear2020Member2021-12-310000230557sigi:PersonalAutomobileMembersigi:StandardPersonalLinesMemberus-gaap:ShortDurationInsuranceContractAccidentYear2021Member2021-12-310000230557sigi:PersonalAutomobileMembersigi:StandardPersonalLinesMember2021-12-310000230557sigi:PersonalAutomobileMembersigi:ShortDurationInsuranceContractsAccidentYearsPriorTo2012Membersigi:StandardPersonalLinesMember2021-12-310000230557sigi:HomeownersMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2012-12-310000230557sigi:HomeownersMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2013-12-310000230557sigi:HomeownersMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2014-12-310000230557sigi:HomeownersMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2015-12-310000230557sigi:HomeownersMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2016-12-310000230557sigi:HomeownersMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2017-12-310000230557sigi:HomeownersMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2018-12-310000230557sigi:HomeownersMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2019-12-310000230557sigi:HomeownersMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2020-12-310000230557sigi:HomeownersMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2021-12-310000230557sigi:HomeownersMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Membersigi:StandardPersonalLinesMember2013-12-310000230557sigi:HomeownersMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Membersigi:StandardPersonalLinesMember2014-12-310000230557sigi:HomeownersMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Membersigi:StandardPersonalLinesMember2015-12-310000230557sigi:HomeownersMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Membersigi:StandardPersonalLinesMember2016-12-310000230557sigi:HomeownersMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Membersigi:StandardPersonalLinesMember2017-12-310000230557sigi:HomeownersMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Membersigi:StandardPersonalLinesMember2018-12-310000230557sigi:HomeownersMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Membersigi:StandardPersonalLinesMember2019-12-310000230557sigi:HomeownersMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Membersigi:StandardPersonalLinesMember2020-12-310000230557sigi:HomeownersMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Membersigi:StandardPersonalLinesMember2021-12-310000230557sigi:HomeownersMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Membersigi:StandardPersonalLinesMember2014-12-310000230557sigi:HomeownersMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Membersigi:StandardPersonalLinesMember2015-12-310000230557sigi:HomeownersMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Membersigi:StandardPersonalLinesMember2016-12-310000230557sigi:HomeownersMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Membersigi:StandardPersonalLinesMember2017-12-310000230557sigi:HomeownersMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Membersigi:StandardPersonalLinesMember2018-12-310000230557sigi:HomeownersMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Membersigi:StandardPersonalLinesMember2019-12-310000230557sigi:HomeownersMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Membersigi:StandardPersonalLinesMember2020-12-310000230557sigi:HomeownersMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Membersigi:StandardPersonalLinesMember2021-12-310000230557sigi:HomeownersMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2015-12-310000230557sigi:HomeownersMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2016-12-310000230557sigi:HomeownersMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2017-12-310000230557sigi:HomeownersMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2018-12-310000230557sigi:HomeownersMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2019-12-310000230557sigi:HomeownersMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2020-12-310000230557sigi:HomeownersMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2021-12-310000230557sigi:HomeownersMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2016-12-310000230557sigi:HomeownersMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2017-12-310000230557sigi:HomeownersMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2018-12-310000230557sigi:HomeownersMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2019-12-310000230557sigi:HomeownersMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2020-12-310000230557sigi:HomeownersMembersigi:StandardPersonalLinesMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2021-12-310000230557sigi:HomeownersMembersigi:StandardPersonalLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Member2017-12-310000230557sigi:HomeownersMembersigi:StandardPersonalLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Member2018-12-310000230557sigi:HomeownersMembersigi:StandardPersonalLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Member2019-12-310000230557sigi:HomeownersMembersigi:StandardPersonalLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Member2020-12-310000230557sigi:HomeownersMembersigi:StandardPersonalLinesMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Member2021-12-310000230557sigi:HomeownersMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2018Membersigi:StandardPersonalLinesMember2018-12-310000230557sigi:HomeownersMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2018Membersigi:StandardPersonalLinesMember2019-12-310000230557sigi:HomeownersMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2018Membersigi:StandardPersonalLinesMember2020-12-310000230557sigi:HomeownersMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2018Membersigi:StandardPersonalLinesMember2021-12-310000230557sigi:HomeownersMemberus-gaap:ShortDurationInsuranceContractAccidentYear2019Membersigi:StandardPersonalLinesMember2019-12-310000230557sigi:HomeownersMemberus-gaap:ShortDurationInsuranceContractAccidentYear2019Membersigi:StandardPersonalLinesMember2020-12-310000230557sigi:HomeownersMemberus-gaap:ShortDurationInsuranceContractAccidentYear2019Membersigi:StandardPersonalLinesMember2021-12-310000230557sigi:HomeownersMembersigi:StandardPersonalLinesMemberus-gaap:ShortDurationInsuranceContractAccidentYear2020Member2020-12-310000230557sigi:HomeownersMembersigi:StandardPersonalLinesMemberus-gaap:ShortDurationInsuranceContractAccidentYear2020Member2021-12-310000230557sigi:HomeownersMembersigi:StandardPersonalLinesMemberus-gaap:ShortDurationInsuranceContractAccidentYear2021Member2021-12-310000230557sigi:HomeownersMembersigi:StandardPersonalLinesMember2021-12-310000230557sigi:HomeownersMembersigi:ShortDurationInsuranceContractsAccidentYearsPriorTo2012Membersigi:StandardPersonalLinesMember2021-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2012-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2013-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2014-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2015-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2016-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2017-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2018-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2019-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2020-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2012Member2021-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2013-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2014-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2015-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2016-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2017-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2018-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2019-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2020-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2013Member2021-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2014-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2015-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2016-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2017-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2018-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2019-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2020-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2014Member2021-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2015-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2016-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2017-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2018-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2019-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2020-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2015Member2021-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2016-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2017-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2018-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2019-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2020-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortdurationInsuranceContractsAccidentYear2016Member2021-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Member2017-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Member2018-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Member2019-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Member2020-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2017Member2021-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2018Member2018-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2018Member2019-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2018Member2020-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortDurationInsuranceContractsAccidentYear2018Member2021-12-310000230557us-gaap:ShortDurationInsuranceContractAccidentYear2019Membersigi:ESLinesMemberus-gaap:GeneralLiabilityMember2019-12-310000230557us-gaap:ShortDurationInsuranceContractAccidentYear2019Membersigi:ESLinesMemberus-gaap:GeneralLiabilityMember2020-12-310000230557us-gaap:ShortDurationInsuranceContractAccidentYear2019Membersigi:ESLinesMemberus-gaap:GeneralLiabilityMember2021-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortDurationInsuranceContractAccidentYear2020Member2020-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortDurationInsuranceContractAccidentYear2020Member2021-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMemberus-gaap:ShortDurationInsuranceContractAccidentYear2021Member2021-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMember2021-12-310000230557sigi:ESLinesMemberus-gaap:GeneralLiabilityMembersigi:ShortDurationInsuranceContractsAccidentYearsPriorTo2012Member2021-12-310000230557us-gaap:GeneralLiabilityMembersigi:StandardCommercialLinesMember2021-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMember2021-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMember2021-12-310000230557sigi:StandardCommercialLinesMembersigi:BusinessOwnersPoliciesMember2021-12-310000230557sigi:CommercialPropertyMembersigi:StandardCommercialLinesMember2021-12-310000230557sigi:StandardCommercialLinesMembersigi:OtherCommercialMember2021-12-310000230557sigi:StandardCommercialLinesMembersigi:TotalCommercialLinesMember2021-12-310000230557sigi:PersonalAutomobileMembersigi:StandardPersonalLinesMember2021-12-310000230557sigi:HomeownersMembersigi:StandardPersonalLinesMember2021-12-310000230557sigi:OtherPersonalMembersigi:StandardPersonalLinesMember2021-12-310000230557sigi:StandardPersonalLinesMembersigi:StandardPersonalLinesMember2021-12-310000230557us-gaap:GeneralLiabilityMembersigi:ESLinesMember2021-12-310000230557sigi:ESLinesMembersigi:StandardCommercialLinesMember2021-12-310000230557sigi:TotalESLinesMembersigi:ESLinesMember2021-12-310000230557sigi:OnePointFiftySixPercentBorrowingFromFederalHomeLoanBankDueAugust162021Member2021-12-310000230557sigi:OnePointSixtyOnePercentBorrowingFromFederalHomeLoanBankDueJuly212021Member2021-12-310000230557sigi:FinanceleaseobligationMember2021-12-310000230557sigi:FinanceleaseobligationMember2020-12-310000230557us-gaap:LineOfCreditMember2021-12-310000230557us-gaap:LineOfCreditMember2021-01-012021-12-310000230557sigi:FivePointThreeHundredSeventyFiveMember2021-01-012021-12-310000230557sigi:FivePointEightHundredSeventyFiveMember2021-12-310000230557sigi:FivePointEightHundredSeventyFiveMember2021-01-012021-12-310000230557srt:FederalHomeLoanBankOfIndianapolisMember2020-12-310000230557srt:FederalHomeLoanBankOfIndianapolisMember2021-12-310000230557sigi:ThreePointZeroThreePercentBorrowingFromFederalHomeLoanBankDueDecember162026Member2020-01-012020-12-310000230557srt:FederalHomeLoanBankOfNewYorkMember2021-12-310000230557srt:FederalHomeLoanBankOfNewYorkMember2020-12-310000230557sigi:SixPointSeventyPercentSeniorNotesMember2005-12-310000230557sigi:SixPointSeventyPercentSeniorNotesMember2005-10-012005-12-310000230557sigi:SevenPointTwentyFivePercentSeniorNotesMember2004-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMember2021-01-012021-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMember2020-01-012020-12-310000230557sigi:CommercialAutomobileMembersigi:StandardCommercialLinesMember2019-01-012019-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMember2021-01-012021-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMember2020-01-012020-12-310000230557us-gaap:WorkersCompensationInsuranceMembersigi:StandardCommercialLinesMember2019-01-012019-12-310000230557us-gaap:GeneralLiabilityMembersigi:StandardCommercialLinesMember2021-01-012021-12-310000230557us-gaap:GeneralLiabilityMembersigi:StandardCommercialLinesMember2020-01-012020-12-310000230557us-gaap:GeneralLiabilityMembersigi:StandardCommercialLinesMember2019-01-012019-12-310000230557sigi:CommercialPropertyMembersigi:StandardCommercialLinesMember2021-01-012021-12-310000230557sigi:CommercialPropertyMembersigi:StandardCommercialLinesMember2020-01-012020-12-310000230557sigi:CommercialPropertyMembersigi:StandardCommercialLinesMember2019-01-012019-12-310000230557sigi:StandardCommercialLinesMembersigi:BusinessOwnersPoliciesMember2021-01-012021-12-310000230557sigi:StandardCommercialLinesMembersigi:BusinessOwnersPoliciesMember2020-01-012020-12-310000230557sigi:StandardCommercialLinesMembersigi:BusinessOwnersPoliciesMember2019-01-012019-12-310000230557us-gaap:SuretySegmentMembersigi:StandardCommercialLinesMember2021-01-012021-12-310000230557us-gaap:SuretySegmentMembersigi:StandardCommercialLinesMember2020-01-012020-12-310000230557us-gaap:SuretySegmentMembersigi:StandardCommercialLinesMember2019-01-012019-12-310000230557sigi:StandardCommercialLinesMembersigi:OtherCommercialMember2021-01-012021-12-310000230557sigi:StandardCommercialLinesMembersigi:OtherCommercialMember2020-01-012020-12-310000230557sigi:StandardCommercialLinesMembersigi:OtherCommercialMember2019-01-012019-12-310000230557sigi:StandardCommercialLinesMember2021-01-012021-12-310000230557sigi:StandardCommercialLinesMember2020-01-012020-12-310000230557sigi:StandardCommercialLinesMember2019-01-012019-12-310000230557sigi:PersonalAutomobileMembersigi:StandardPersonalLinesMember2021-01-012021-12-310000230557sigi:PersonalAutomobileMembersigi:StandardPersonalLinesMember2020-01-012020-12-310000230557sigi:PersonalAutomobileMembersigi:StandardPersonalLinesMember2019-01-012019-12-310000230557sigi:HomeownersMembersigi:StandardPersonalLinesMember2021-01-012021-12-310000230557sigi:HomeownersMembersigi:StandardPersonalLinesMember2020-01-012020-12-310000230557sigi:HomeownersMembersigi:StandardPersonalLinesMember2019-01-012019-12-310000230557sigi:OtherPersonalMembersigi:StandardPersonalLinesMember2021-01-012021-12-310000230557sigi:OtherPersonalMembersigi:StandardPersonalLinesMember2020-01-012020-12-310000230557sigi:OtherPersonalMembersigi:StandardPersonalLinesMember2019-01-012019-12-310000230557sigi:StandardPersonalLinesMember2021-01-012021-12-310000230557sigi:StandardPersonalLinesMember2020-01-012020-12-310000230557sigi:StandardPersonalLinesMember2019-01-012019-12-310000230557sigi:ESLinesMembersigi:CasualtyInsuranceProductLineMember2021-01-012021-12-310000230557sigi:ESLinesMembersigi:CasualtyInsuranceProductLineMember2020-01-012020-12-310000230557sigi:ESLinesMembersigi:CasualtyInsuranceProductLineMember2019-01-012019-12-310000230557sigi:ESLinesMemberus-gaap:PropertyInsuranceProductLineMember2021-01-012021-12-310000230557sigi:ESLinesMemberus-gaap:PropertyInsuranceProductLineMember2020-01-012020-12-310000230557sigi:ESLinesMemberus-gaap:PropertyInsuranceProductLineMember2019-01-012019-12-310000230557sigi:ESLinesMember2021-01-012021-12-310000230557sigi:ESLinesMember2020-01-012020-12-310000230557sigi:ESLinesMember2019-01-012019-12-310000230557sigi:InvestmentsSIGIMember2021-01-012021-12-310000230557sigi:InvestmentsSIGIMember2020-01-012020-12-310000230557sigi:InvestmentsSIGIMember2019-01-012019-12-310000230557sigi:OperatingSegmentsSIGIMember2021-01-012021-12-310000230557sigi:OperatingSegmentsSIGIMember2020-01-012020-12-310000230557sigi:OperatingSegmentsSIGIMember2019-01-012019-12-310000230557sigi:DeferredTaxLiabilityMember2021-12-310000230557sigi:DeferredTaxLiabilityMember2020-12-310000230557us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember2021-01-012021-12-310000230557us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember2020-01-012020-12-310000230557us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember2019-01-012019-12-310000230557us-gaap:PensionPlansDefinedBenefitMember2020-12-310000230557us-gaap:PensionPlansDefinedBenefitMember2019-12-310000230557us-gaap:PensionPlansDefinedBenefitMember2021-01-012021-12-310000230557us-gaap:PensionPlansDefinedBenefitMember2020-01-012020-12-310000230557us-gaap:PensionPlansDefinedBenefitMember2021-12-310000230557us-gaap:PensionPlansDefinedBenefitMember2019-01-012019-12-310000230557srt:MinimumMembersigi:ReturnSeekingAssetsMember2021-01-012021-12-310000230557srt:MaximumMembersigi:ReturnSeekingAssetsMember2021-01-012021-12-310000230557sigi:ReturnSeekingAssetsMember2021-12-310000230557sigi:ReturnSeekingAssetsMember2020-12-310000230557srt:MinimumMembersigi:LiabilityHedgingAssetsMember2021-01-012021-12-310000230557srt:MaximumMembersigi:LiabilityHedgingAssetsMember2021-01-012021-12-310000230557sigi:LiabilityHedgingAssetsMember2021-12-310000230557sigi:LiabilityHedgingAssetsMember2020-12-310000230557srt:MinimumMemberus-gaap:ShortTermInvestmentsMember2021-01-012021-12-310000230557srt:MaximumMemberus-gaap:ShortTermInvestmentsMember2021-01-012021-12-310000230557us-gaap:ShortTermInvestmentsMember2021-12-310000230557us-gaap:ShortTermInvestmentsMember2020-12-310000230557srt:MinimumMember2021-12-310000230557srt:MaximumMember2021-12-310000230557us-gaap:PensionPlansDefinedBenefitMembersigi:GlobalEquityMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:PensionPlansDefinedBenefitMembersigi:GlobalEquityMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:PensionPlansDefinedBenefitMembersigi:GlobalEquityMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel3Membersigi:GlobalEquityMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557sigi:DiversifiedCreditMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557sigi:DiversifiedCreditMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:FairValueInputsLevel2Membersigi:DiversifiedCreditMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557sigi:DiversifiedCreditMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel3Memberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:PensionPlansDefinedBenefitMembersigi:RealAssetsEquityMemberMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel1Membersigi:RealAssetsEquityMemberMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:PensionPlansDefinedBenefitMembersigi:RealAssetsEquityMemberMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel3Membersigi:RealAssetsEquityMemberMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557sigi:TotalEquitiesMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557sigi:TotalEquitiesMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:FairValueInputsLevel2Membersigi:TotalEquitiesMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557sigi:TotalEquitiesMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel3Memberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMembersigi:RealAssetsMember2021-12-310000230557us-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMembersigi:RealAssetsMember2021-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMembersigi:RealAssetsMember2021-12-310000230557us-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel3Memberus-gaap:FairValueMeasurementsRecurringMembersigi:RealAssetsMember2021-12-310000230557us-gaap:PrivateEquityFundsMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:PrivateEquityFundsMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:PrivateEquityFundsMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:PrivateEquityFundsMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel3Memberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557srt:PartnershipInterestMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel1Membersrt:PartnershipInterestMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:PensionPlansDefinedBenefitMembersrt:PartnershipInterestMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel3Membersrt:PartnershipInterestMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557sigi:ReturnSeekingAssetsMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557sigi:ReturnSeekingAssetsMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:FairValueInputsLevel2Membersigi:ReturnSeekingAssetsMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557sigi:ReturnSeekingAssetsMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel3Memberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:FixedIncomeSecuritiesMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:FixedIncomeSecuritiesMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:FixedIncomeSecuritiesMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:FixedIncomeSecuritiesMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel3Memberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557sigi:USTreasuryFundMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557sigi:USTreasuryFundMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:FairValueInputsLevel2Membersigi:USTreasuryFundMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557sigi:USTreasuryFundMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel3Memberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:PensionPlansDefinedBenefitMembersigi:LiabilityHedgingAssetsMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:PensionPlansDefinedBenefitMembersigi:LiabilityHedgingAssetsMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:PensionPlansDefinedBenefitMembersigi:LiabilityHedgingAssetsMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel3Membersigi:LiabilityHedgingAssetsMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMemberus-gaap:ShortTermInvestmentsMember2021-12-310000230557us-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMemberus-gaap:ShortTermInvestmentsMember2021-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMemberus-gaap:ShortTermInvestmentsMember2021-12-310000230557us-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel3Memberus-gaap:FairValueMeasurementsRecurringMemberus-gaap:ShortTermInvestmentsMember2021-12-310000230557us-gaap:PensionPlansDefinedBenefitMembersigi:DepositAdministrationContractsMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:PensionPlansDefinedBenefitMembersigi:DepositAdministrationContractsMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:PensionPlansDefinedBenefitMembersigi:DepositAdministrationContractsMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel3Membersigi:DepositAdministrationContractsMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557sigi:CashAndShortTermInvestmentsMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557sigi:CashAndShortTermInvestmentsMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:FairValueInputsLevel2Membersigi:CashAndShortTermInvestmentsMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557sigi:CashAndShortTermInvestmentsMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel3Memberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel3Memberus-gaap:FairValueMeasurementsRecurringMember2021-12-310000230557us-gaap:PensionPlansDefinedBenefitMembersigi:GlobalEquityMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:PensionPlansDefinedBenefitMembersigi:GlobalEquityMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:PensionPlansDefinedBenefitMembersigi:GlobalEquityMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel3Membersigi:GlobalEquityMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557sigi:DiversifiedCreditMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557sigi:DiversifiedCreditMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:FairValueInputsLevel2Membersigi:DiversifiedCreditMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557sigi:DiversifiedCreditMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel3Memberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:PensionPlansDefinedBenefitMembersigi:RealAssetsEquityMemberMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel1Membersigi:RealAssetsEquityMemberMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:PensionPlansDefinedBenefitMembersigi:RealAssetsEquityMemberMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel3Membersigi:RealAssetsEquityMemberMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557sigi:TotalEquitiesMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557sigi:TotalEquitiesMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:FairValueInputsLevel2Membersigi:TotalEquitiesMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557sigi:TotalEquitiesMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel3Memberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMembersigi:RealAssetsMember2020-12-310000230557us-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMembersigi:RealAssetsMember2020-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMembersigi:RealAssetsMember2020-12-310000230557us-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel3Memberus-gaap:FairValueMeasurementsRecurringMembersigi:RealAssetsMember2020-12-310000230557us-gaap:PrivateEquityFundsMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:PrivateEquityFundsMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:PrivateEquityFundsMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:PrivateEquityFundsMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel3Memberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557sigi:PrivateCreditFundsMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557sigi:PrivateCreditFundsMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:FairValueInputsLevel2Membersigi:PrivateCreditFundsMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557sigi:PrivateCreditFundsMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel3Memberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557srt:PartnershipInterestMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel1Membersrt:PartnershipInterestMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:PensionPlansDefinedBenefitMembersrt:PartnershipInterestMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel3Membersrt:PartnershipInterestMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557sigi:ReturnSeekingAssetsMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557sigi:ReturnSeekingAssetsMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:FairValueInputsLevel2Membersigi:ReturnSeekingAssetsMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557sigi:ReturnSeekingAssetsMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel3Memberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:FixedIncomeSecuritiesMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:FixedIncomeSecuritiesMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:FixedIncomeSecuritiesMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:FixedIncomeSecuritiesMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel3Memberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557sigi:USTreasuryFundMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557sigi:USTreasuryFundMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:FairValueInputsLevel2Membersigi:USTreasuryFundMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557sigi:USTreasuryFundMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel3Memberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:PensionPlansDefinedBenefitMembersigi:LiabilityHedgingAssetsMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:PensionPlansDefinedBenefitMembersigi:LiabilityHedgingAssetsMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:PensionPlansDefinedBenefitMembersigi:LiabilityHedgingAssetsMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel3Membersigi:LiabilityHedgingAssetsMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMemberus-gaap:ShortTermInvestmentsMember2020-12-310000230557us-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMemberus-gaap:ShortTermInvestmentsMember2020-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMemberus-gaap:ShortTermInvestmentsMember2020-12-310000230557us-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel3Memberus-gaap:FairValueMeasurementsRecurringMemberus-gaap:ShortTermInvestmentsMember2020-12-310000230557us-gaap:PensionPlansDefinedBenefitMembersigi:DepositAdministrationContractsMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:PensionPlansDefinedBenefitMembersigi:DepositAdministrationContractsMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:PensionPlansDefinedBenefitMembersigi:DepositAdministrationContractsMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel3Membersigi:DepositAdministrationContractsMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557sigi:CashAndShortTermInvestmentsMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557sigi:CashAndShortTermInvestmentsMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:FairValueInputsLevel2Membersigi:CashAndShortTermInvestmentsMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557sigi:CashAndShortTermInvestmentsMemberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel3Memberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel1Memberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:FairValueInputsLevel2Memberus-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557us-gaap:PensionPlansDefinedBenefitMemberus-gaap:FairValueInputsLevel3Memberus-gaap:FairValueMeasurementsRecurringMember2020-12-310000230557sigi:OmnibusStockPlan2014Member2021-12-310000230557sigi:EmployeeStockPurchasePlanMember2021-12-310000230557sigi:AgentStockPurchasePlanMember2021-12-310000230557sigi:Omnibus2005StockPlanMember2021-12-310000230557sigi:StockCompensationPlanForNonEmployeeDirectorsMember2021-12-310000230557us-gaap:RestrictedStockUnitsRSUMember2020-12-310000230557us-gaap:RestrictedStockUnitsRSUMember2021-01-012021-12-310000230557us-gaap:RestrictedStockUnitsRSUMember2021-12-310000230557us-gaap:RestrictedStockUnitsRSUMember2020-01-012020-12-310000230557us-gaap:RestrictedStockUnitsRSUMember2019-01-012019-12-310000230557sigi:DividendEquivalentUnitsMember2021-01-012021-12-310000230557sigi:DividendEquivalentUnitsMember2020-01-012020-12-310000230557sigi:DividendEquivalentUnitsMember2019-01-012019-12-310000230557sigi:CashIncentivePlanMember2021-12-310000230557sigi:CashIncentivePlanMember2020-12-310000230557sigi:CashIncentivePlanMember2021-01-012021-12-310000230557sigi:CashIncentivePlanMember2020-01-012020-12-310000230557sigi:CashIncentivePlanMember2019-01-012019-12-310000230557sigi:EmployeeStockPurchasePlanMember2021-01-012021-12-310000230557sigi:EmployeeStockPurchasePlanMember2020-01-012020-12-310000230557sigi:EmployeeStockPurchasePlanMember2019-01-012019-12-310000230557sigi:AgentStockPurchasePlanMember2021-01-012021-12-310000230557sigi:AgentStockPurchasePlanMember2020-01-012020-12-310000230557sigi:AgentStockPurchasePlanMember2019-01-012019-12-310000230557us-gaap:EmployeeStockMember2021-01-012021-12-310000230557us-gaap:EmployeeStockMember2020-01-012020-12-310000230557us-gaap:EmployeeStockMember2019-01-012019-12-310000230557sigi:AgentStockPurchasePlanMember2021-01-012021-12-310000230557sigi:AgentStockPurchasePlanMember2020-01-012020-12-310000230557sigi:AgentStockPurchasePlanMember2019-01-012019-12-310000230557us-gaap:SeriesAMember2020-12-310000230557us-gaap:SeriesAMember2021-12-310000230557us-gaap:SeriesBMember2021-01-012021-12-310000230557sigi:ProceedsFromPreferredStockOfferingLessIssuanceCostsMember2021-01-012021-12-310000230557sigi:RatingAgencyEventMember2021-12-310000230557sigi:RegulatoryCapitalEventMember2021-12-310000230557sigi:RelatedPartyInsuranceAgencyMember2021-12-310000230557sigi:RelatedPartyInsuranceAgencyMember2021-01-012021-12-310000230557sigi:RelatedPartyInsuranceAgencyMember2020-01-012020-12-310000230557sigi:RelatedPartyInsuranceAgencyMember2019-01-012019-12-310000230557sigi:RelatedPartyInsuranceAgencyMember2020-12-310000230557sigi:BlackRockIncMember2021-12-310000230557sigi:BlackRockIncMember2021-01-012021-12-310000230557sigi:BlackRockIncMember2020-01-012020-12-310000230557sigi:BlackRockIncMember2019-01-012019-12-310000230557us-gaap:PensionPlansDefinedBenefitMembersigi:BlackRockIncMember2021-12-310000230557us-gaap:PensionPlansDefinedBenefitMembersigi:BlackRockIncMember2020-12-310000230557us-gaap:PensionPlansDefinedBenefitMembersigi:BlackRockIncMember2021-01-012021-12-310000230557us-gaap:PensionPlansDefinedBenefitMembersigi:BlackRockIncMember2020-01-012020-12-310000230557us-gaap:PensionPlansDefinedBenefitMembersigi:BlackRockIncMember2019-01-012019-12-310000230557us-gaap:OtherLiabilitiesMember2021-12-310000230557us-gaap:OtherLiabilitiesMember2020-12-310000230557us-gaap:OtherAggregatedInvestmentsMember2021-12-310000230557us-gaap:CollateralizedLoanObligationsMember2021-12-310000230557us-gaap:CommonStockMember2021-12-310000230557sigi:CommercialMortgageLoansSIGIMember2021-12-310000230557sigi:PrivatePlacementCorporateSecuritiesMember2021-12-310000230557us-gaap:InvestmentsMember2021-12-310000230557sigi:SelectiveInsuranceCompanyOfAmericaMember2021-12-310000230557sigi:SelectiveInsuranceCompanyOfAmericaMember2020-12-310000230557sigi:SelectiveInsuranceCompanyOfAmericaMember2021-01-012021-12-310000230557sigi:SelectiveInsuranceCompanyOfAmericaMember2020-01-012020-12-310000230557sigi:SelectiveInsuranceCompanyOfAmericaMember2019-01-012019-12-310000230557sigi:SelectiveWayInsuranceCompanyMember2021-12-310000230557sigi:SelectiveWayInsuranceCompanyMember2020-12-310000230557sigi:SelectiveWayInsuranceCompanyMember2021-01-012021-12-310000230557sigi:SelectiveWayInsuranceCompanyMember2020-01-012020-12-310000230557sigi:SelectiveWayInsuranceCompanyMember2019-01-012019-12-310000230557sigi:SelectiveInsuranceCompanyOfSouthCarolinaMember2021-12-310000230557sigi:SelectiveInsuranceCompanyOfSouthCarolinaMember2020-12-310000230557sigi:SelectiveInsuranceCompanyOfSouthCarolinaMember2021-01-012021-12-310000230557sigi:SelectiveInsuranceCompanyOfSouthCarolinaMember2020-01-012020-12-310000230557sigi:SelectiveInsuranceCompanyOfSouthCarolinaMember2019-01-012019-12-310000230557sigi:SelectiveInsuranceCompanyOfSoutheastMember2021-12-310000230557sigi:SelectiveInsuranceCompanyOfSoutheastMember2020-12-310000230557sigi:SelectiveInsuranceCompanyOfSoutheastMember2021-01-012021-12-310000230557sigi:SelectiveInsuranceCompanyOfSoutheastMember2020-01-012020-12-310000230557sigi:SelectiveInsuranceCompanyOfSoutheastMember2019-01-012019-12-310000230557sigi:SelectiveInsuranceCompanyOfNewYorkMember2021-12-310000230557sigi:SelectiveInsuranceCompanyOfNewYorkMember2020-12-310000230557sigi:SelectiveInsuranceCompanyOfNewYorkMember2021-01-012021-12-310000230557sigi:SelectiveInsuranceCompanyOfNewYorkMember2020-01-012020-12-310000230557sigi:SelectiveInsuranceCompanyOfNewYorkMember2019-01-012019-12-310000230557sigi:SelectiveInsuranceCompanyOfNewEnglandMember2021-12-310000230557sigi:SelectiveInsuranceCompanyOfNewEnglandMember2020-12-310000230557sigi:SelectiveInsuranceCompanyOfNewEnglandMember2021-01-012021-12-310000230557sigi:SelectiveInsuranceCompanyOfNewEnglandMember2020-01-012020-12-310000230557sigi:SelectiveInsuranceCompanyOfNewEnglandMember2019-01-012019-12-310000230557sigi:SelectiveAutoInsuranceCompanyOfNewJerseyMember2021-12-310000230557sigi:SelectiveAutoInsuranceCompanyOfNewJerseyMember2020-12-310000230557sigi:SelectiveAutoInsuranceCompanyOfNewJerseyMember2021-01-012021-12-310000230557sigi:SelectiveAutoInsuranceCompanyOfNewJerseyMember2020-01-012020-12-310000230557sigi:SelectiveAutoInsuranceCompanyOfNewJerseyMember2019-01-012019-12-310000230557sigi:MesaUnderwritingSpecialtyInsuranceCompanyMember2021-12-310000230557sigi:MesaUnderwritingSpecialtyInsuranceCompanyMember2020-12-310000230557sigi:MesaUnderwritingSpecialtyInsuranceCompanyMember2021-01-012021-12-310000230557sigi:MesaUnderwritingSpecialtyInsuranceCompanyMember2020-01-012020-12-310000230557sigi:MesaUnderwritingSpecialtyInsuranceCompanyMember2019-01-012019-12-310000230557sigi:SelectiveCasualtyInsuranceCompanyMember2021-12-310000230557sigi:SelectiveCasualtyInsuranceCompanyMember2020-12-310000230557sigi:SelectiveCasualtyInsuranceCompanyMember2021-01-012021-12-310000230557sigi:SelectiveCasualtyInsuranceCompanyMember2020-01-012020-12-310000230557sigi:SelectiveCasualtyInsuranceCompanyMember2019-01-012019-12-310000230557sigi:SelectiveFireAndCasualtyInsuranceCompanyMember2021-12-310000230557sigi:SelectiveFireAndCasualtyInsuranceCompanyMember2020-12-310000230557sigi:SelectiveFireAndCasualtyInsuranceCompanyMember2021-01-012021-12-310000230557sigi:SelectiveFireAndCasualtyInsuranceCompanyMember2020-01-012020-12-310000230557sigi:SelectiveFireAndCasualtyInsuranceCompanyMember2019-01-012019-12-310000230557srt:ParentCompanyMember2021-12-310000230557srt:ParentCompanyMember2021-01-012021-12-310000230557sigi:SelectiveInsuranceCompanyOfAmericaMember2021-01-012021-12-310000230557sigi:SelectiveInsuranceCompanyOfAmericaMember2021-12-310000230557sigi:SelectiveWayInsuranceCompanyMember2021-01-012021-12-310000230557sigi:SelectiveWayInsuranceCompanyMember2021-12-310000230557sigi:SelectiveInsuranceCompanyOfSouthCarolinaMember2021-01-012021-12-310000230557sigi:SelectiveInsuranceCompanyOfSouthCarolinaMember2021-12-310000230557sigi:SelectiveInsuranceCompanyOfSoutheastMember2021-01-012021-12-310000230557sigi:SelectiveInsuranceCompanyOfSoutheastMember2021-12-310000230557sigi:SelectiveInsuranceCompanyOfNewYorkMember2021-01-012021-12-310000230557sigi:SelectiveInsuranceCompanyOfNewYorkMember2021-12-310000230557sigi:SelectiveInsuranceCompanyOfNewEnglandMember2021-01-012021-12-310000230557sigi:SelectiveInsuranceCompanyOfNewEnglandMember2021-12-310000230557sigi:SelectiveAutoInsuranceCompanyOfNewJerseyMember2021-01-012021-12-310000230557sigi:SelectiveAutoInsuranceCompanyOfNewJerseyMember2021-12-310000230557sigi:MesaUnderwritingSpecialtyInsuranceCompanyMember2021-01-012021-12-310000230557sigi:MesaUnderwritingSpecialtyInsuranceCompanyMember2021-12-310000230557sigi:SelectiveCasualtyInsuranceCompanyMember2021-01-012021-12-310000230557sigi:SelectiveCasualtyInsuranceCompanyMember2021-12-310000230557sigi:SelectiveFireAndCasualtyInsuranceCompanyMember2021-01-012021-12-310000230557sigi:SelectiveFireAndCasualtyInsuranceCompanyMember2021-12-310000230557us-gaap:HeldtomaturitySecuritiesMemberus-gaap:USStatesAndPoliticalSubdivisionsMember2021-12-310000230557us-gaap:HeldtomaturitySecuritiesMemberus-gaap:PublicUtilityBondsMember2021-12-310000230557us-gaap:HeldtomaturitySecuritiesMemberus-gaap:AllOtherCorporateBondsMember2021-12-310000230557us-gaap:HeldtomaturitySecuritiesMemberus-gaap:FixedMaturitiesMember2021-12-310000230557us-gaap:USGovernmentAgenciesDebtSecuritiesMembersigi:AFSFixedIncomeSecuritiesMember2021-12-310000230557us-gaap:ForeignGovernmentDebtMembersigi:AFSFixedIncomeSecuritiesMember2021-12-310000230557us-gaap:USStatesAndPoliticalSubdivisionsMembersigi:AFSFixedIncomeSecuritiesMember2021-12-310000230557sigi:AFSFixedIncomeSecuritiesMemberus-gaap:PublicUtilityBondsMember2021-12-310000230557us-gaap:AllOtherCorporateBondsMembersigi:AFSFixedIncomeSecuritiesMember2021-12-310000230557us-gaap:MortgageBackedSecuritiesMembersigi:AFSFixedIncomeSecuritiesMember2021-12-310000230557us-gaap:CommercialMortgageBackedSecuritiesMembersigi:AFSFixedIncomeSecuritiesMember2021-12-310000230557sigi:AFSFixedIncomeSecuritiesMemberus-gaap:ResidentialMortgageBackedSecuritiesMember2021-12-310000230557sigi:AFSFixedIncomeSecuritiesMemberus-gaap:FixedMaturitiesMember2021-12-310000230557us-gaap:BanksTrustAndInsuranceEquitiesMemberus-gaap:EquitySecuritiesMember2021-12-310000230557us-gaap:IndustrialMiscellaneousAndAllOthersMemberus-gaap:EquitySecuritiesMember2021-12-310000230557us-gaap:PreferredStockMemberus-gaap:EquitySecuritiesMember2021-12-310000230557us-gaap:EquitySecuritiesMemberus-gaap:EquitySecuritiesMember2021-12-310000230557sigi:CommercialMortgageLoansSIGIMember2021-12-310000230557srt:ParentCompanyMember2021-12-310000230557srt:ParentCompanyMember2020-12-310000230557srt:ParentCompanyMember2021-01-012021-12-310000230557srt:ParentCompanyMember2020-01-012020-12-310000230557srt:ParentCompanyMember2019-01-012019-12-310000230557srt:ParentCompanyMember2019-12-310000230557srt:ParentCompanyMember2018-12-310000230557sigi:StandardCommercialLinesMember2021-12-310000230557sigi:StandardPersonalLinesMember2021-12-310000230557sigi:ESLinesMember2021-12-310000230557sigi:InvestmentsSIGIMember2021-12-310000230557sigi:MiscellaneousIncomeMember2021-01-012021-12-310000230557sigi:StandardCommercialLinesMember2020-12-310000230557sigi:StandardPersonalLinesMember2020-12-310000230557sigi:ESLinesMember2020-12-310000230557sigi:InvestmentsSIGIMember2020-12-310000230557sigi:MiscellaneousIncomeMember2020-01-012020-12-310000230557sigi:StandardCommercialLinesMember2019-12-310000230557sigi:StandardPersonalLinesMember2019-12-310000230557sigi:ESLinesMember2019-12-310000230557sigi:InvestmentsSIGIMember2019-12-310000230557sigi:MiscellaneousIncomeMember2019-01-012019-12-310000230557us-gaap:AccidentAndHealthInsuranceSegmentMember2021-01-012021-12-310000230557us-gaap:PropertyLiabilityAndCasualtyInsuranceSegmentMember2021-01-012021-12-310000230557us-gaap:AccidentAndHealthInsuranceSegmentMember2020-01-012020-12-310000230557us-gaap:PropertyLiabilityAndCasualtyInsuranceSegmentMember2020-01-012020-12-310000230557us-gaap:AccidentAndHealthInsuranceSegmentMember2019-01-012019-12-310000230557us-gaap:PropertyLiabilityAndCasualtyInsuranceSegmentMember2019-01-012019-12-310000230557us-gaap:AccountingStandardsUpdate201613Member2020-01-010000230557us-gaap:AccountingStandardsUpdate201613Member2018-12-310000230557us-gaap:PremiumsReceivableMemberus-gaap:AccountingStandardsUpdate201613Member2020-12-310000230557us-gaap:PremiumsReceivableMemberus-gaap:AccountingStandardsUpdate201613Member2019-12-310000230557us-gaap:PremiumsReceivableMemberus-gaap:AccountingStandardsUpdate201613Member2018-12-31

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, DC 20549

 

FORM 10-K

(Mark One)

 

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

 

For the fiscal year ended: December 31, 2021

or

 

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

 

For the transition period from_______________________to_______________________

Commission file number: 001-33067 

SELECTIVE INSURANCE GROUP, INC.

(Exact Name of Registrant as Specified in Its Charter)

New Jersey						22-2168890
(State or Other Jurisdiction of Incorporation or Organization)						(I.R.S. Employer Identification No.)

40 Wantage Avenue

Branchville, New Jersey 07890

(Address of Principal Executive Offices) (Zip Code)

973			948-3000
(Registrant's Telephone Number, Including Area Code)

 

Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days. ☒ Yes    ☐ No

Indicate by check mark whether the registrant has submitted electronically every Interactive Data File required to be submitted pursuant to Rule 405 of Regulation S-T during the preceding 12 months (or for such shorter period that the registrant was required to submit such files). ☒ Yes     ☐ No

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, a smaller reporting company, or an emerging growth company. See the definitions of “large accelerated filer,” “accelerated filer,” “smaller reporting company,” and "emerging growth company" in Rule 12b-2 of the Exchange Act.

 

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act.

                                                     ☐

Indicate by check mark whether the registrant has filed a report on and attestation to its management's assessment of the effectiveness of its internal control over financial reporting under Section 404(b) of the Sarbanes-Oxley Act (15 U.S.C. 7262(b)) by the registered public accounting firm that prepared or issued its audit report.                  ☒

Indicate by check mark whether the registrant is a shell company (as defined in Rule 12b-2 of the Act). ☐ Yes   ☒ No 

The aggregate market value of the voting company common stock held by non-affiliates of the registrant, based on the closing price on the Nasdaq Global Select Market, was $4,787,831,538 on June 30, 2021. As of January 31, 2022, the registrant had outstanding 60,186,063 shares of common stock.

 

DOCUMENTS INCORPORATED BY REFERENCE

Portions of the registrant’s definitive Proxy Statement for the 2022 Annual Meeting of Stockholders to be held on May 3, 2022, are incorporated by reference into Part III of this report.

			SELECTIVE INSURANCE GROUP, INC.
			Table of Contents
						Page No.
PART I
Item 1.			Business			4
Item 1A.			Risk Factors			20
Item 1B.			Unresolved Staff Comments			32
Item 2.			Properties			32
Item 3.			Legal Proceedings			32
Item 4.			Mine Safety Disclosures			32
PART II
Item 5.			Market For Registrant’s Common Equity, Related Stockholder Matters and Issuer Purchases of Equity Securities			32
Item 6.			Reserved			34
Item 7.			Management’s Discussion and Analysis of Financial Condition and Results of Operations			34
			Forward-looking Statements			34
			Introduction			34
			Critical Accounting Policies and Estimates			35
			Financial Highlights of Results for Years Ended December 31, 2021, 2020, and 2019			43
			Results of Operations and Related Information by Segment			46
			Federal Income Taxes			59
			Liquidity and Capital Resources			59
Item 7A.			Quantitative and Qualitative Disclosures About Market Risk			63
Item 8.			Financial Statements and Supplementary Data			70
			Consolidated Balance Sheets as of December 31, 2021 and 2020			72
			Consolidated Statements of Income for the Years Ended
			December 31, 2021, 2020, and 2019			73
			Consolidated Statements of Comprehensive Income for the Years Ended
			December 31, 2021, 2020, and 2019			74
			Consolidated Statements of Stockholders’ Equity for the Years Ended
			December 31, 2021, 2020, and 2019			75
			Consolidated Statements of Cash Flows for the Years Ended
			December 31, 2021, 2020, and 2019			76
			Notes to Consolidated Financial Statements			77
Item 9.			Changes in and Disagreements With Accountants on Accounting and Financial Disclosure			127
Item 9A.			Controls and Procedures			127
Item 9B.			Other Information			129
Item 9C.			Disclosure Regarding Foreign Jurisdictions that Prevent Inspections			129
PART III
Item 10.			Directors, Executive Officers and Corporate Governance			129
Item 11.			Executive Compensation			129
Item 12.			Security Ownership of Certain Beneficial Owners and Management and Related Stockholder Matters			129
Item 13.			Certain Relationships and Related Transactions, and Director Independence			129
Item 14.			Principal Accountant Fees and Services			130
PART IV
Item 15.			Exhibit and Financial Statement Schedules			130
Item 16.			Form 10-K Summary			141