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Stockholders' Equity and Comprehensive Income (Reclassification out of AOCI) (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Other Comprehensive Income Loss Reclassification Adjustment From AOCI For Non Credit Other Than Temporary Impaired Securities Before Tax $ 1,669sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesBeforeTax $ 14sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesBeforeTax $ 280sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesBeforeTax
Total HTM Related, Tax (455)sigi_AmortizationOfNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleTax (552)sigi_AmortizationOfNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleTax (851)sigi_AmortizationOfNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleTax
Amount reclassified from AOCI, HTM (844)sigi_AmortizationofNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleNetOfTax (1,025)sigi_AmortizationofNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleNetOfTax (1,581)sigi_AmortizationofNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleNetOfTax
Total realized gains and losses on AFS disposals, gross (28,864)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax (23,540)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax (9,412)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax
Realized gains and losses on AFS disposals, Tax 10,102us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax 8,239us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax 3,294us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax
Realized gains on available for sale securities (18,762)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax (15,301)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax (6,118)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, for Net Gain (Loss), before Tax 1,902us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax 4,374us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax 5,903us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax
Prior service cost   10us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax 150us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax
Curtailment Expense   16sigi_OtherComprehensiveIncomeLossAmortizationPensionAndOtherPostretirementBenefitPlansCurtailmentExpenseRecognizedInNetPeriodicBenefitCostBeforeTax  
Total defined benefit pension and post-retirement life, Gross (52,234)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentBeforeTax 64,054us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentBeforeTax (20,513)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentBeforeTax
Total defined benefit pension and post-retirement life,Tax 18,281us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansTax (22,419)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansTax 7,179us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansTax
Total defined benefit pension and post-retirement plans (33,953)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax 41,635us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax (13,334)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax
Policy acquisition costs [Member]      
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, for Net Gain (Loss), before Tax 1,571us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_PolicyAcquisitionCostsMember
3,465us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_PolicyAcquisitionCostsMember
 
Prior service cost 0us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_PolicyAcquisitionCostsMember
3us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_PolicyAcquisitionCostsMember
 
Curtailment Expense 0sigi_OtherComprehensiveIncomeLossAmortizationPensionAndOtherPostretirementBenefitPlansCurtailmentExpenseRecognizedInNetPeriodicBenefitCostBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_PolicyAcquisitionCostsMember
16sigi_OtherComprehensiveIncomeLossAmortizationPensionAndOtherPostretirementBenefitPlansCurtailmentExpenseRecognizedInNetPeriodicBenefitCostBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_PolicyAcquisitionCostsMember
 
Loss and loss expense incurred [Member]      
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, for Net Gain (Loss), before Tax 331us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_LossAndLossExpenseIncurredMember
909us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_LossAndLossExpenseIncurredMember
 
Prior service cost 0us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_LossAndLossExpenseIncurredMember
7us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_LossAndLossExpenseIncurredMember
 
Net Realized Investment Gains [Member]      
Unrealized gains and losses on HTM disposals 157sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentForUnrealizedGainsAndLossesOnHeldToMaturityDisposalsBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_NetRealizedInvestmentGainsMember
390sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentForUnrealizedGainsAndLossesOnHeldToMaturityDisposalsBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_NetRealizedInvestmentGainsMember
 
Total realized gains and losses on AFS disposals, gross (28,864)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_NetRealizedInvestmentGainsMember
(23,540)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_NetRealizedInvestmentGainsMember
 
Net Income Financial Statement Line Item [Member]      
Amount reclassified from AOCI (17,285)us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_NetIncomeFinancialStatementLineItemMember
(13,457)us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_NetIncomeFinancialStatementLineItemMember
 
Amount reclassified from AOCI, HTM (844)sigi_AmortizationofNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleNetOfTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_NetIncomeFinancialStatementLineItemMember
(1,025)sigi_AmortizationofNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleNetOfTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_NetIncomeFinancialStatementLineItemMember
 
Realized gains on available for sale securities (18,762)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_NetIncomeFinancialStatementLineItemMember
(15,301)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_NetIncomeFinancialStatementLineItemMember
 
Net Income Financial Statement Line Item [Member] | Accumulated Defined Benefit Plans Adjustment [Member]      
Total defined benefit pension and post-retirement plans 1,236us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_NetIncomeFinancialStatementLineItemMember
/ us-gaap_StatementScenarioAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
2,860us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_NetIncomeFinancialStatementLineItemMember
/ us-gaap_StatementScenarioAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
 
Net Income Financial Statement Line Item [Member] | Other-than-Temporary Impairment [Member]      
Amount reclassified from AOCI 1,085us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_NetIncomeFinancialStatementLineItemMember
/ us-gaap_StatementScenarioAxis
= us-gaap_AccumulatedOtherThanTemporaryImpairmentMember
9us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_NetIncomeFinancialStatementLineItemMember
/ us-gaap_StatementScenarioAxis
= us-gaap_AccumulatedOtherThanTemporaryImpairmentMember
 
Total Federal Income Tax Expense [Member]      
Total HTM Related, Tax 455sigi_AmortizationOfNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_TotalFederalIncomeTaxExpenseMember
552sigi_AmortizationOfNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_TotalFederalIncomeTaxExpenseMember
 
Realized gains and losses on AFS disposals, Tax 10,102us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_TotalFederalIncomeTaxExpenseMember
8,239us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_TotalFederalIncomeTaxExpenseMember
 
Total defined benefit pension and post-retirement life,Tax (666)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_TotalFederalIncomeTaxExpenseMember
(1,540)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_TotalFederalIncomeTaxExpenseMember
 
Total Federal Income Tax Expense [Member] | Other-than-Temporary Impairment [Member]      
Amortization of non-credit OTTI losses on HTM securities, tax (584)sigi_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrrentPeriodTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_TotalFederalIncomeTaxExpenseMember
/ us-gaap_StatementScenarioAxis
= us-gaap_AccumulatedOtherThanTemporaryImpairmentMember
(5)sigi_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrrentPeriodTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_TotalFederalIncomeTaxExpenseMember
/ us-gaap_StatementScenarioAxis
= us-gaap_AccumulatedOtherThanTemporaryImpairmentMember
 
Income From Continuing Operations Before Federal Income Tax [Member]      
Other Comprehensive Income Loss Reclassification Adjustment From AOCI For Non Credit Other Than Temporary Impaired Securities Before Tax 1,669sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_IncomeFromContinuingOperationsBeforeFederalIncomeTaxMember
14sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_IncomeFromContinuingOperationsBeforeFederalIncomeTaxMember
 
Amortization on net unrealized gains on HTM securities (1,299)sigi_AmortizationOfNetUnrealizedGainsOnHTMSecurities
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_IncomeFromContinuingOperationsBeforeFederalIncomeTaxMember
(1,577)sigi_AmortizationOfNetUnrealizedGainsOnHTMSecurities
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_IncomeFromContinuingOperationsBeforeFederalIncomeTaxMember
 
Total realized gains and losses on AFS disposals, gross (28,864)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_IncomeFromContinuingOperationsBeforeFederalIncomeTaxMember
(23,540)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_IncomeFromContinuingOperationsBeforeFederalIncomeTaxMember
 
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, for Net Gain (Loss), before Tax 1,902us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_IncomeFromContinuingOperationsBeforeFederalIncomeTaxMember
4,374us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_IncomeFromContinuingOperationsBeforeFederalIncomeTaxMember
 
Prior service cost 0us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_IncomeFromContinuingOperationsBeforeFederalIncomeTaxMember
10us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_IncomeFromContinuingOperationsBeforeFederalIncomeTaxMember
 
Curtailment Expense 0sigi_OtherComprehensiveIncomeLossAmortizationPensionAndOtherPostretirementBenefitPlansCurtailmentExpenseRecognizedInNetPeriodicBenefitCostBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_IncomeFromContinuingOperationsBeforeFederalIncomeTaxMember
16sigi_OtherComprehensiveIncomeLossAmortizationPensionAndOtherPostretirementBenefitPlansCurtailmentExpenseRecognizedInNetPeriodicBenefitCostBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_IncomeFromContinuingOperationsBeforeFederalIncomeTaxMember
 
Total defined benefit pension and post-retirement life, Gross 1,902us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_IncomeFromContinuingOperationsBeforeFederalIncomeTaxMember
4,400us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_IncomeFromContinuingOperationsBeforeFederalIncomeTaxMember
 
Net Investment Income Earned [Member]      
Amortization on net unrealized gains on HTM securities (1,456)sigi_AmortizationOfNetUnrealizedGainsOnHTMSecurities
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_NetInvestmentIncomeEarnedMember
(1,967)sigi_AmortizationOfNetUnrealizedGainsOnHTMSecurities
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_NetInvestmentIncomeEarnedMember
 
Amortization Of Non Credit OTTI Losses On HTM Securities [Member] | Net Investment Income Earned [Member]      
Other Comprehensive Income Loss Reclassification Adjustment From AOCI For Non Credit Other Than Temporary Impaired Securities Before Tax 0sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_NetInvestmentIncomeEarnedMember
/ us-gaap_StatementEquityComponentsAxis
= sigi_AmortizationOfNonCreditOTTILossesOnHTMSecuritiesMember
14sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_NetInvestmentIncomeEarnedMember
/ us-gaap_StatementEquityComponentsAxis
= sigi_AmortizationOfNonCreditOTTILossesOnHTMSecuritiesMember
 
Non Credit OTTI On Disposed Securities [Member] | Net Realized Investment Gains [Member]      
Other Comprehensive Income Loss Reclassification Adjustment From AOCI For Non Credit Other Than Temporary Impaired Securities Before Tax $ 1,669sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_NetRealizedInvestmentGainsMember
/ us-gaap_StatementEquityComponentsAxis
= sigi_NonCreditOTTIOnDisposedSecuritiesMember
$ 0sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_NetRealizedInvestmentGainsMember
/ us-gaap_StatementEquityComponentsAxis
= sigi_NonCreditOTTIOnDisposedSecuritiesMember