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v2.4.1.9

Comprehensive Income (Reclassification out of AOCI) (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Amounts reclassified into net income: non-credit OTTI | $ 357 sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesBeforeTax |
Non-credit OTTI, Tax | (125)sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesTax |
Amounts reclassified into Net Income: Non-credit other-than-temporary impairments | 232 sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesNetOfTax | 0 sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesNetOfTax
Reclassification of net unrealized gains on HTM securities including disposals and amortization | (261)sigi_AmortizationOfNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleBeforeTax | (456)sigi_AmortizationOfNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleBeforeTax
Total HTM Related, Tax | 91 sigi_AmortizationOfNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleTax | 160 sigi_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 | 6,963 us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax | 2,530 us-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 | 1,709 us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax | 380 us-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 | 1,111 us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax | 247 us-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 | 357 sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesBeforeTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_IncomeFromContinuingOperationsBeforeFederalIncomeTaxMember | 0 sigi_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 | 1,709 us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_IncomeFromContinuingOperationsBeforeFederalIncomeTaxMember | 380 us-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 | 0 sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_TotalFederalIncomeTaxExpenseMember
Total HTM Related, Tax | 91 sigi_AmortizationOfNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_TotalFederalIncomeTaxExpenseMember | 160 sigi_AmortizationOfNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_TotalFederalIncomeTaxExpenseMember
Total realized gains on AFS disposals and OTTI | 6,963 us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_TotalFederalIncomeTaxExpenseMember | 2,530 us-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 | 232 sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesNetOfTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_NetIncomeFinancialStatementLineItemMember | 0 sigi_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 | 1,111 us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_NetIncomeFinancialStatementLineItemMember | 247 us-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 | 50 sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentForUnrealizedGainsAndLossesOnHeldToMaturityDisposalsBeforeTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_NetRealizedInvestmentGainsMember | 24 sigi_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 | 357 sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesBeforeTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_NetRealizedInvestmentGainsMember / us-gaap_StatementEquityComponentsAxis = sigi_NonCreditOTTIOnDisposedSecuritiesMember | 0 sigi_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 | 371 us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_LossAndLossExpenseIncurredMember | 87 us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_LossAndLossExpenseIncurredMember
Policy acquisition costs [Member] |  |
Amount reclassified into Net Income: Net actuarial loss, Gross | $ 1,338 us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_PolicyAcquisitionCostsMember | $ 293 us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_PolicyAcquisitionCostsMember

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