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v2.4.1.9

Stockholders' Equity and Comprehensive Income (Reclassification out of AOCI) (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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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 | $ 1,669 sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesBeforeTax | $ 14 sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesBeforeTax | $ 280 sigi_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 | 10,102 us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax | 8,239 us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax | 3,294 us-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 | 1,902 us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax | 4,374 us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax | 5,903 us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax
Prior service cost |  | 10 us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax | 150 us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax
Curtailment Expense |  | 16 sigi_OtherComprehensiveIncomeLossAmortizationPensionAndOtherPostretirementBenefitPlansCurtailmentExpenseRecognizedInNetPeriodicBenefitCostBeforeTax |
Total defined benefit pension and post-retirement life, Gross | (52,234)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentBeforeTax | 64,054 us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentBeforeTax | (20,513)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentBeforeTax
Total defined benefit pension and post-retirement life,Tax | 18,281 us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansTax | (22,419)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansTax | 7,179 us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansTax
Total defined benefit pension and post-retirement plans | (33,953)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax | 41,635 us-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 | 1,571 us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_PolicyAcquisitionCostsMember | 3,465 us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_PolicyAcquisitionCostsMember |
Prior service cost | 0 us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_PolicyAcquisitionCostsMember | 3 us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_PolicyAcquisitionCostsMember |
Curtailment Expense | 0 sigi_OtherComprehensiveIncomeLossAmortizationPensionAndOtherPostretirementBenefitPlansCurtailmentExpenseRecognizedInNetPeriodicBenefitCostBeforeTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_PolicyAcquisitionCostsMember | 16 sigi_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 | 331 us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_LossAndLossExpenseIncurredMember | 909 us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_LossAndLossExpenseIncurredMember |
Prior service cost | 0 us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_LossAndLossExpenseIncurredMember | 7 us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_LossAndLossExpenseIncurredMember |
Net Realized Investment Gains [Member] |  |  |
Unrealized gains and losses on HTM disposals | 157 sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentForUnrealizedGainsAndLossesOnHeldToMaturityDisposalsBeforeTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_NetRealizedInvestmentGainsMember | 390 sigi_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 | 1,236 us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_NetIncomeFinancialStatementLineItemMember / us-gaap_StatementScenarioAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember | 2,860 us-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 | 1,085 us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_NetIncomeFinancialStatementLineItemMember / us-gaap_StatementScenarioAxis = us-gaap_AccumulatedOtherThanTemporaryImpairmentMember | 9 us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_NetIncomeFinancialStatementLineItemMember / us-gaap_StatementScenarioAxis = us-gaap_AccumulatedOtherThanTemporaryImpairmentMember |
Total Federal Income Tax Expense [Member] |  |  |
Total HTM Related, Tax | 455 sigi_AmortizationOfNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_TotalFederalIncomeTaxExpenseMember | 552 sigi_AmortizationOfNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_TotalFederalIncomeTaxExpenseMember |
Realized gains and losses on AFS disposals, Tax | 10,102 us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_TotalFederalIncomeTaxExpenseMember | 8,239 us-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 | 1,669 sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesBeforeTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_IncomeFromContinuingOperationsBeforeFederalIncomeTaxMember | 14 sigi_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 | 1,902 us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_IncomeFromContinuingOperationsBeforeFederalIncomeTaxMember | 4,374 us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_IncomeFromContinuingOperationsBeforeFederalIncomeTaxMember |
Prior service cost | 0 us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_IncomeFromContinuingOperationsBeforeFederalIncomeTaxMember | 10 us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_IncomeFromContinuingOperationsBeforeFederalIncomeTaxMember |
Curtailment Expense | 0 sigi_OtherComprehensiveIncomeLossAmortizationPensionAndOtherPostretirementBenefitPlansCurtailmentExpenseRecognizedInNetPeriodicBenefitCostBeforeTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_IncomeFromContinuingOperationsBeforeFederalIncomeTaxMember | 16 sigi_OtherComprehensiveIncomeLossAmortizationPensionAndOtherPostretirementBenefitPlansCurtailmentExpenseRecognizedInNetPeriodicBenefitCostBeforeTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_IncomeFromContinuingOperationsBeforeFederalIncomeTaxMember |
Total defined benefit pension and post-retirement life, Gross | 1,902 us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentBeforeTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_IncomeFromContinuingOperationsBeforeFederalIncomeTaxMember | 4,400 us-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 | 0 sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesBeforeTax / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = sigi_NetInvestmentIncomeEarnedMember / us-gaap_StatementEquityComponentsAxis = sigi_AmortizationOfNonCreditOTTILossesOnHTMSecuritiesMember | 14 sigi_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 | $ 1,669 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 |

X
- Definition Net of tax amount of the reclassification adjustment relaated to held-to-maturity securities transferred from the available-for-sale category. This amount includes amortization and amounts realized from the disposal of these securities. + References No definition available. + Details
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- Definition Tax effect of the reclassification adjustment related to held-to-maturity securities transferred from the available-for-sale category. This amount includes amortization and amounts realized from the disposal of these securities. + References No definition available. + Details
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- Definition Before tax amount of the income statement impact of the reclassification adjustment for amortization of net unrealized holding gain or loss of held-to-maturity securities transferred from the available-for-sale category + References No definition available. + Details
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- Definition The amount of net gain (loss) reclassified from Other Comprehensive Income recognized in net periodic benefit cost as a result of an event that significantly reduces the expected years of future service of present employees or eliminates for a significant number of employees the accrual of defined benefits for some or all of their future services. + References No definition available. + Details
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- Definition Before tax amount of the income statement impact of the reclassification adjustment for unrealized gain (loss) realized upon the disposal of held-to-matuirty securities that were transferred from an-available-for-sale designation. + References No definition available. + Details
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- Definition Amount before tax of reclassification adjustment from accumulated other comprehensive income related to non-credit OTTI securities. + References No definition available. + Details
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- Definition Amount before tax of reclassification adjustment from accumulated other comprehensive (income) loss for prior service cost (credit) related to pension and other postretirement benefit plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=36458714&loc=d3e637-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=36458714&loc=SL7669646-108580 + Details
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- Definition Amount before tax, after reclassification adjustments, of (increase) decrease in accumulated other comprehensive (income) loss related to pension and other postretirement defined benefit plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (i-k) -URI http://asc.fasb.org/extlink&oid=36458714&loc=SL7669646-108580 + Details
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- Definition Amount after tax and reclassification adjustments, of (increase) decrease in accumulated other comprehensive (income) loss related to pension and other postretirement defined benefit plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (i-k) -URI http://asc.fasb.org/extlink&oid=36458714&loc=SL7669646-108580 + Details
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- Definition Amount of tax (expense) benefit, after reclassification adjustments, of (increase) decrease in accumulated other comprehensive (income) loss related to pension and other postretirement defined benefit plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=36458714&loc=d3e640-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (i-k) -URI http://asc.fasb.org/extlink&oid=36458714&loc=SL7669646-108580 + Details
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