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Stockholders' Equity and Comprehensive Income (Components of Comprehensive Income-Gross and Net of Tax) (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 12 Months Ended
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2013
Sep. 30, 2013
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Income Loss From Continuing Operations And Discontinued Operations Before Federal Income Tax                   $ 142,267sigi_IncomeLossFromContinuingOperationsAndDiscontinuedOperationsBeforeFederalIncomeTax  
Net Income, Gross                 197,131us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest 143,802us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest 37,635us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest
Income From Continuing Operations And Discontinued Operations Tax                   35,849sigi_IncomeFromContinuingOperationsAndDiscontinuedOperationsTax  
Net Income, Tax                 55,304us-gaap_IncomeTaxExpenseBenefit 36,387us-gaap_IncomeTaxExpenseBenefit (328)us-gaap_IncomeTaxExpenseBenefit
Net income 41,350us-gaap_NetIncomeLoss 53,162us-gaap_NetIncomeLoss 29,341us-gaap_NetIncomeLoss 17,974us-gaap_NetIncomeLoss 25,335us-gaap_NetIncomeLoss 32,653us-gaap_NetIncomeLoss 27,122us-gaap_NetIncomeLoss 21,308us-gaap_NetIncomeLoss 141,827us-gaap_NetIncomeLoss 106,418us-gaap_NetIncomeLoss 37,963us-gaap_NetIncomeLoss
Unrealized holding gains (losses) during the period, Gross                 72,940us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodBeforeTax (83,934)us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodBeforeTax 47,594us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodBeforeTax
Unrealized holding gains (losses) during the period, Tax                 25,529us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodTax (29,377)us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodTax 16,657us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodTax
Unrealized holding gains (losses) arising during period                 47,411us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax (54,557)us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax 30,937us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax
Non-credit portion of other-than-temporary impairment losses recognized in other comprehensive income, Gross                   77sigi_NonCreditPortionOfOtherThanTemporaryImpairmentLossesRecognizedInOtherComprehensiveIncomeGross 2,554sigi_NonCreditPortionOfOtherThanTemporaryImpairmentLossesRecognizedInOtherComprehensiveIncomeGross
Non-credit portion of other-than-temporary impairment losses recognized in other comprehensive income, Tax                   27us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossTaxPortionAttributableToParent 894us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossTaxPortionAttributableToParent
Non-credit portion of other-than-temporary impairments recognized in other comprehensive income                   50us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent 1,660us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent
Amount reclassified into net income: HTM securities, Gross                 (1,299)sigi_AmortizationOfNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleBeforeTax (1,577)sigi_AmortizationOfNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleBeforeTax (2,432)sigi_AmortizationOfNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleBeforeTax
Amounts reclassified into net income: HTM securities, 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
Non-credit OTTI, Gross                 1,669sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesBeforeTax 14sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesBeforeTax 280sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesBeforeTax
Non-credit OTTI, Tax                 584sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesTax 5sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesTax 98sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesTax
Amounts reclassified into net income: Non-credit other-than-temporary impairment                 1,085sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesNetOfTax 9sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesNetOfTax 182sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesNetOfTax
Realized gains on AFS securities, Gross                 (28,864)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax (23,540)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax (9,412)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax
Realized gains on AFS securities, 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
Net unrealized gains (losses), Gross                 44,446sigi_TotalUnrealizedGainsLossesOnInvestmentSecuritiesGross (108,960)sigi_TotalUnrealizedGainsLossesOnInvestmentSecuritiesGross 38,584sigi_TotalUnrealizedGainsLossesOnInvestmentSecuritiesGross
Net unrealized gains (losses), Tax                 15,556sigi_TotalUnrealizedGainsLossesOnInvestmentSecuritiesTax (38,136)sigi_TotalUnrealizedGainsLossesOnInvestmentSecuritiesTax 13,504sigi_TotalUnrealizedGainsLossesOnInvestmentSecuritiesTax
Total unrealized gains (losses) on investment securities                 28,890sigi_TotalUnrealizedGainsLossesonInvestmentSecurities (70,824)sigi_TotalUnrealizedGainsLossesonInvestmentSecurities 25,080sigi_TotalUnrealizedGainsLossesonInvestmentSecurities
Net actuarial gain (loss), Gross                 (54,136)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansNetUnamortizedGainLossArisingDuringPeriodBeforeTax 59,654us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansNetUnamortizedGainLossArisingDuringPeriodBeforeTax (26,566)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansNetUnamortizedGainLossArisingDuringPeriodBeforeTax
Net actuarial gain (loss), Tax                 (18,947)us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodTax 20,879us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodTax (9,298)us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodTax
Net actuarial (loss) gain                 (35,189)us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodNetOfTax 38,775us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodNetOfTax (17,268)us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodNetOfTax
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
Amount reclassified into Net Income: Net actuarial loss, Tax                 666us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossTax 1,531us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossTax 2,066us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossTax
Amount reclassified into net income: Net actuarial loss                 1,236us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossNetOfTax 2,843us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossNetOfTax 3,837us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossNetOfTax
Amount reclassified into Net Income: Prior service cost, Gross                   10us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax 150us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax
Amount reclassified into Net Income: Prior service cost, Tax                   4us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditTax 53us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditTax
Amount reclassified into net income: Prior service cost                 0us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditNetOfTax 6us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditNetOfTax 97us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditNetOfTax
Amount reclassified into Net Income: Curtailment expense, Gross                   16sigi_OtherComprehensiveIncomeLossAmortizationPensionAndOtherPostretirementBenefitPlansCurtailmentExpenseRecognizedInNetPeriodicBenefitCostBeforeTax  
Amount reclassified into Net Income: Curtailment expense, Tax                   5sigi_OtherComprehensiveIncomeLossAmortizationPensionAndOtherPostretirementBenefitPlansCurtailmentExpenseRecognizedInNetPeriodicBenefitCostTax  
Amount reclassified into net income: Curtailment expense, Net of tax                   11sigi_OtherComprehensiveIncomeLossAmortizationPensionAndOtherPostRetirementBenefitPlansCurtailmentExpenseRecognizedInNetPeriodicBenefitCostNetofTax  
Defined pension and other post-retirement benefit plans, Gross                 (52,234)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentBeforeTax 64,054us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentBeforeTax (20,513)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentBeforeTax
Defined pension and other post-retirement benefit plans, Tax                 (18,281)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansTax 22,419us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansTax (7,179)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansTax
Total defined benefit pension and post-retirement plans                 (33,953)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax 41,635us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax (13,334)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax
Other Comprehensive (Loss) Income, Gross                 (7,788)us-gaap_OtherComprehensiveIncomeLossBeforeTax (44,906)us-gaap_OtherComprehensiveIncomeLossBeforeTax 18,071us-gaap_OtherComprehensiveIncomeLossBeforeTax
Other Comprehensive (Loss) Income, Tax                 (2,725)us-gaap_OtherComprehensiveIncomeLossTax (15,717)us-gaap_OtherComprehensiveIncomeLossTax 6,325us-gaap_OtherComprehensiveIncomeLossTax
Other comprehensive (loss) income (29,337)us-gaap_OtherComprehensiveIncomeLossNetOfTax (18,887)us-gaap_OtherComprehensiveIncomeLossNetOfTax 26,483us-gaap_OtherComprehensiveIncomeLossNetOfTax 16,678us-gaap_OtherComprehensiveIncomeLossNetOfTax 7,768us-gaap_OtherComprehensiveIncomeLossNetOfTax (2,195)us-gaap_OtherComprehensiveIncomeLossNetOfTax (62,643)us-gaap_OtherComprehensiveIncomeLossNetOfTax 27,881us-gaap_OtherComprehensiveIncomeLossNetOfTax (5,063)us-gaap_OtherComprehensiveIncomeLossNetOfTax (29,189)us-gaap_OtherComprehensiveIncomeLossNetOfTax 11,746us-gaap_OtherComprehensiveIncomeLossNetOfTax
Comprehensive income, Gross                 189,343sigi_ComprehensiveIncomeLossBeforeTax 97,361sigi_ComprehensiveIncomeLossBeforeTax 55,706sigi_ComprehensiveIncomeLossBeforeTax
Comprehensive income, Tax                 52,579sigi_ComprehensiveIncomeLossTax 20,132sigi_ComprehensiveIncomeLossTax 5,997sigi_ComprehensiveIncomeLossTax
Comprehensive income 12,013us-gaap_ComprehensiveIncomeNetOfTax 34,275us-gaap_ComprehensiveIncomeNetOfTax 55,824us-gaap_ComprehensiveIncomeNetOfTax 34,652us-gaap_ComprehensiveIncomeNetOfTax 33,103us-gaap_ComprehensiveIncomeNetOfTax 30,458us-gaap_ComprehensiveIncomeNetOfTax (35,521)us-gaap_ComprehensiveIncomeNetOfTax 49,189us-gaap_ComprehensiveIncomeNetOfTax 136,764us-gaap_ComprehensiveIncomeNetOfTax 77,229us-gaap_ComprehensiveIncomeNetOfTax 49,709us-gaap_ComprehensiveIncomeNetOfTax
Accumulated Other-than-Temporary Impairment [Member]                      
Other comprehensive (loss) income                 1,085us-gaap_OtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherThanTemporaryImpairmentMember		 59us-gaap_OtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherThanTemporaryImpairmentMember		  
Accumulated Defined Benefit Plans Adjustment [Member]                      
Net actuarial gain (loss), Gross               44,000us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansNetUnamortizedGainLossArisingDuringPeriodBeforeTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember		      
Total defined benefit pension and post-retirement plans                 1,236us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember		 2,860us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember		  
Accumulated other comprehensive income (loss) [Member]                      
Other comprehensive (loss) income                 $ (5,063)us-gaap_OtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember		 $ (29,189)us-gaap_OtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember		 $ 11,746us-gaap_OtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember