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Comprehensive Income (Reclassification out of AOCI) (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Amounts reclassified into net income: non-credit OTTI $ 357sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesBeforeTax  
Non-credit OTTI, Tax (125)sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesTax  
Amounts reclassified into Net Income: Non-credit other-than-temporary impairments 232sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesNetOfTax 0sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesNetOfTax
Reclassification of net unrealized gains on HTM securities including disposals and amortization (261)sigi_AmortizationOfNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleBeforeTax (456)sigi_AmortizationOfNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleBeforeTax
Total HTM Related, Tax 91sigi_AmortizationOfNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleTax 160sigi_AmortizationOfNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleTax
Amounts reclassified from AOCI, HTM Related (170)sigi_AmortizationofNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleNetofTax (296)sigi_AmortizationofNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleNetofTax
Realized gains on AFS disposals and OTTI (19,895)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax (7,229)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax
Total realized gains on AFS disposals and OTTI 6,963us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax 2,530us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax
Amounts reclassified into net income: realized gains on available for sale securities (12,932)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax (4,699)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax
Amount reclassified into Net Income: Net actuarial loss, Gross 1,709us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax 380us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax
Defined pension and other post-retirement benefit plans, tax (598)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansTax (133)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansTax
Total defined benefit pension and post-retirement plans 1,111us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax 247us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax
Investment Income [Member]    
Amortization of net unrealized gains on HTM securities (311)sigi_AmortizationOfNetUnrealizedGainsOnHTMSecurities
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= us-gaap_InvestmentIncomeMember
(480)sigi_AmortizationOfNetUnrealizedGainsOnHTMSecurities
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= us-gaap_InvestmentIncomeMember
Income Before Federal Income Tax [Member]    
Amounts reclassified into net income: non-credit OTTI 357sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_IncomeFromContinuingOperationsBeforeFederalIncomeTaxMember
0sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_IncomeFromContinuingOperationsBeforeFederalIncomeTaxMember
Reclassification of net unrealized gains on HTM securities including disposals and amortization (261)sigi_AmortizationOfNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_IncomeFromContinuingOperationsBeforeFederalIncomeTaxMember
(456)sigi_AmortizationOfNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_IncomeFromContinuingOperationsBeforeFederalIncomeTaxMember
Realized gains on AFS disposals and OTTI (19,895)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_IncomeFromContinuingOperationsBeforeFederalIncomeTaxMember
(7,229)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_IncomeFromContinuingOperationsBeforeFederalIncomeTaxMember
Amount reclassified into Net Income: Net actuarial loss, Gross 1,709us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_IncomeFromContinuingOperationsBeforeFederalIncomeTaxMember
380us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_IncomeFromContinuingOperationsBeforeFederalIncomeTaxMember
Total Federal Income Tax Expense [Member]    
Non-credit OTTI, Tax (125)sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_TotalFederalIncomeTaxExpenseMember
0sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_TotalFederalIncomeTaxExpenseMember
Total HTM Related, Tax 91sigi_AmortizationOfNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_TotalFederalIncomeTaxExpenseMember
160sigi_AmortizationOfNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_TotalFederalIncomeTaxExpenseMember
Total realized gains on AFS disposals and OTTI 6,963us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_TotalFederalIncomeTaxExpenseMember
2,530us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_TotalFederalIncomeTaxExpenseMember
Defined pension and other post-retirement benefit plans, tax (598)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_TotalFederalIncomeTaxExpenseMember
(133)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_TotalFederalIncomeTaxExpenseMember
Net Income Financial Statement Line Item [Member]    
Amounts reclassified into Net Income: Non-credit other-than-temporary impairments 232sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesNetOfTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_NetIncomeFinancialStatementLineItemMember
0sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesNetOfTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_NetIncomeFinancialStatementLineItemMember
Amounts reclassified from AOCI, HTM Related (170)sigi_AmortizationofNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleNetofTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_NetIncomeFinancialStatementLineItemMember
(296)sigi_AmortizationofNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleNetofTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_NetIncomeFinancialStatementLineItemMember
Amounts reclassified into net income: realized gains on available for sale securities (12,932)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_NetIncomeFinancialStatementLineItemMember
(4,699)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_NetIncomeFinancialStatementLineItemMember
Total defined benefit pension and post-retirement plans 1,111us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_NetIncomeFinancialStatementLineItemMember
247us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_NetIncomeFinancialStatementLineItemMember
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax (11,759)us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_NetIncomeFinancialStatementLineItemMember
(4,748)us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_NetIncomeFinancialStatementLineItemMember
Net Realized Investment Gains [Member]    
Unrealized gains and losses on HTM disposals 50sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentForUnrealizedGainsAndLossesOnHeldToMaturityDisposalsBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_NetRealizedInvestmentGainsMember
24sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentForUnrealizedGainsAndLossesOnHeldToMaturityDisposalsBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_NetRealizedInvestmentGainsMember
Realized gains on AFS disposals and OTTI (19,895)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_NetRealizedInvestmentGainsMember
(7,229)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_NetRealizedInvestmentGainsMember
Net Realized Investment Gains [Member] | Non-credit OTTI on Disposed Securities [Member]    
Amounts reclassified into net income: non-credit OTTI 357sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_NetRealizedInvestmentGainsMember
/ us-gaap_StatementEquityComponentsAxis
= sigi_NonCreditOTTIOnDisposedSecuritiesMember
0sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_NetRealizedInvestmentGainsMember
/ us-gaap_StatementEquityComponentsAxis
= sigi_NonCreditOTTIOnDisposedSecuritiesMember
Loss And Loss Expense Incurred [Member]    
Amount reclassified into Net Income: Net actuarial loss, Gross 371us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_LossAndLossExpenseIncurredMember
87us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_LossAndLossExpenseIncurredMember
Policy acquisition costs [Member]    
Amount reclassified into Net Income: Net actuarial loss, Gross $ 1,338us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_PolicyAcquisitionCostsMember
$ 293us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= sigi_PolicyAcquisitionCostsMember