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Investments (Fair Value and Gross Pre-Tax Net Unrealized Unrecognized Loss of Securities By Length of Time) (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
AFS Fair Value, Less than 12 months $ 371,381us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue $ 466,387us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
AFS Unrealized Losses, Less than 12 months (4,884)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss [1] (2,408)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss [1]
AFS Fair Value, 12 months or longer 156,812us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue 442,058us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
AFS Unrealized Losses, 12 months or longer (1,873)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss [1] (8,170)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss [1]
HTM Fair Value, Less than 12 months 197us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue 196us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
HTM Unrealized Losses, Less than 12 months (2)us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss [1] (3)us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss [1]
HTM Unrecognized Gains, Less than 12 months 1sigi_HeldToMaturitySecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionLessThanTwelveMonths [2] 1sigi_HeldToMaturitySecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionLessThanTwelveMonths [2]
HTM Fair Value, 12 months or longer 2,003us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue 2,235us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
HTM Unrealized Losses, 12 months or longer (379)us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss [1] (455)us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss [1]
HTM Unrecognized Gains, 12 months or longer 361sigi_HeldToMaturitySecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionTwelveMonthsorLonger [2] 439sigi_HeldToMaturitySecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionTwelveMonthsorLonger [2]
Total AFS and HTM Fair Value, Less than 12 months 371,578sigi_AvailableForSaleandHeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue 466,583sigi_AvailableForSaleandHeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Total AFS and HTM Unrealized Losses, Less than 12 months (4,886)sigi_AvailableForSaleAndHeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsAggregateLosses [1] (2,411)sigi_AvailableForSaleAndHeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsAggregateLosses [1]
Total AFS and HTM Securities Unrecognized Gains, Less Than 12 Months 1sigi_HeldToMaturityAndAvailableForSaleSecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionLessThanTwelveMonths [2] 1sigi_HeldToMaturityAndAvailableForSaleSecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionLessThanTwelveMonths [2]
Total AFS and HTM Fair Value, 12 months or longer 158,815sigi_AvailableForSaleandHeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue 444,293sigi_AvailableForSaleandHeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Total AFS and HTM Unrealized Losses, 12 months or longer (2,252)sigi_AvailableForSaleAndHeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerAggregateLosses [1] (8,625)sigi_AvailableForSaleAndHeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerAggregateLosses [1]
Total AFS and HTM Securities Unrecognized Gains, 12 months or longer 361sigi_HeldToMaturityAndAvailableForSaleSecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionForTwelveMonthsOrLonger [2] 439sigi_HeldToMaturityAndAvailableForSaleSecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionForTwelveMonthsOrLonger [2]
Obligations of States and Political Subdivisions [Member]    
HTM Fair Value, Less than 12 months 197us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 196us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
HTM Unrealized Losses, Less than 12 months (2)us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 [1] (3)us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 [1]
HTM Unrecognized Gains, Less than 12 months 1sigi_HeldToMaturitySecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionLessThanTwelveMonths
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 [2] 1sigi_HeldToMaturitySecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionLessThanTwelveMonths
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 [2]
HTM Fair Value, 12 months or longer 0us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 0us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
HTM Unrealized Losses, 12 months or longer 0us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 [1] 0us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 [1]
HTM Unrecognized Gains, 12 months or longer 0sigi_HeldToMaturitySecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionTwelveMonthsorLonger
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 [2] 0sigi_HeldToMaturitySecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionTwelveMonthsorLonger
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 [2]
Asset-Backed Securities [Member]    
HTM Fair Value, Less than 12 months 0us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_AssetBackedSecuritiesMember		 0us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_AssetBackedSecuritiesMember
HTM Unrealized Losses, Less than 12 months 0us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_AssetBackedSecuritiesMember		 [1] 0us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_AssetBackedSecuritiesMember		 [1]
HTM Unrecognized Gains, Less than 12 months 0sigi_HeldToMaturitySecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionLessThanTwelveMonths
/ us-gaap_InvestmentTypeAxis
= us-gaap_AssetBackedSecuritiesMember		 [2] 0sigi_HeldToMaturitySecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionLessThanTwelveMonths
/ us-gaap_InvestmentTypeAxis
= us-gaap_AssetBackedSecuritiesMember		 [2]
HTM Fair Value, 12 months or longer 2,003us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_AssetBackedSecuritiesMember		 2,235us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_AssetBackedSecuritiesMember
HTM Unrealized Losses, 12 months or longer (379)us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_AssetBackedSecuritiesMember		 [1] (455)us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_AssetBackedSecuritiesMember		 [1]
HTM Unrecognized Gains, 12 months or longer 361sigi_HeldToMaturitySecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionTwelveMonthsorLonger
/ us-gaap_InvestmentTypeAxis
= us-gaap_AssetBackedSecuritiesMember		 [2] 439sigi_HeldToMaturitySecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionTwelveMonthsorLonger
/ us-gaap_InvestmentTypeAxis
= us-gaap_AssetBackedSecuritiesMember		 [2]
AFS Fixed Income Securities [Member]    
AFS Fair Value, Less than 12 months 271,172us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 461,125us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember
AFS Unrealized Losses, Less than 12 months (1,615)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 [1] (2,072)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 [1]
AFS Fair Value, 12 months or longer 156,812us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 442,058us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember
AFS Unrealized Losses, 12 months or longer (1,873)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 [1] (8,170)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 [1]
AFS Fixed Income Securities [Member] | U.S. Government and Government Agencies [Member]    
AFS Fair Value, Less than 12 months 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_USTreasuryAndGovernmentMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 7,567us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_USTreasuryAndGovernmentMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember
AFS Unrealized Losses, Less than 12 months 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_USTreasuryAndGovernmentMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 [1] (13)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_USTreasuryAndGovernmentMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 [1]
AFS Fair Value, 12 months or longer 398us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_USTreasuryAndGovernmentMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 10,866us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_USTreasuryAndGovernmentMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember
AFS Unrealized Losses, 12 months or longer (2)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_USTreasuryAndGovernmentMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 [1] (115)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_USTreasuryAndGovernmentMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 [1]
AFS Fixed Income Securities [Member] | Obligations of States and Political Subdivisions [Member]    
AFS Fair Value, Less than 12 months 75,935us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 47,510us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember
AFS Unrealized Losses, Less than 12 months (534)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 [1] (105)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 [1]
AFS Fair Value, 12 months or longer 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 64,018us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember
AFS Unrealized Losses, 12 months or longer 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 [1] (624)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 [1]
AFS Fixed Income Securities [Member] | Corporate Securities [Member]    
AFS Fair Value, Less than 12 months 93,912us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateDebtSecuritiesMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 276,648us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateDebtSecuritiesMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember
AFS Unrealized Losses, Less than 12 months (821)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateDebtSecuritiesMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 [1] (1,734)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateDebtSecuritiesMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 [1]
AFS Fair Value, 12 months or longer 50,858us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateDebtSecuritiesMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 153,613us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateDebtSecuritiesMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember
AFS Unrealized Losses, 12 months or longer (718)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateDebtSecuritiesMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 [1] (4,075)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateDebtSecuritiesMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 [1]
AFS Fixed Income Securities [Member] | Asset-Backed Securities [Member]    
AFS Fair Value, Less than 12 months 25,684us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_AssetBackedSecuritiesMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 113,202us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_AssetBackedSecuritiesMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember
AFS Unrealized Losses, Less than 12 months (16)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_AssetBackedSecuritiesMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 [1] (178)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_AssetBackedSecuritiesMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 [1]
AFS Fair Value, 12 months or longer 14,353us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_AssetBackedSecuritiesMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 15,618us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_AssetBackedSecuritiesMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember
AFS Unrealized Losses, 12 months or longer (89)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_AssetBackedSecuritiesMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 [1] (195)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_AssetBackedSecuritiesMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 [1]
AFS Fixed Income Securities [Member] | Commercial Mortgage Backed Securities [Member]    
AFS Fair Value, Less than 12 months 24,664us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 12,799us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember
AFS Unrealized Losses, Less than 12 months (55)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 [1] (34)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 [1]
AFS Fair Value, 12 months or longer 15,682us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 59,219us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember
AFS Unrealized Losses, 12 months or longer (73)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 [1] (743)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 [1]
AFS Fixed Income Securities [Member] | Residential Mortgage Backed Securities [Member]    
AFS Fair Value, Less than 12 months 50,977us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 3,399us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember
AFS Unrealized Losses, Less than 12 months (189)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 [1] (8)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 [1]
AFS Fair Value, 12 months or longer 75,521us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 138,724us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember
AFS Unrealized Losses, 12 months or longer (991)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 [1] (2,418)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= sigi_AFSFixedIncomeSecuritiesMember		 [1]
Equity Securities [Member]    
AFS Fair Value, Less than 12 months 100,209us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= us-gaap_EquitySecuritiesMember		 5,262us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= us-gaap_EquitySecuritiesMember
AFS Unrealized Losses, Less than 12 months (3,269)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= us-gaap_EquitySecuritiesMember		 [1] (336)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= us-gaap_EquitySecuritiesMember		 [1]
AFS Fair Value, 12 months or longer 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= us-gaap_EquitySecuritiesMember		 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= us-gaap_EquitySecuritiesMember
AFS Unrealized Losses, 12 months or longer $ 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= us-gaap_EquitySecuritiesMember		 [1] $ 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis
= us-gaap_EquitySecuritiesMember		 [1]
[1] Gross unrealized losses include non-OTTI unrealized amounts and OTTI losses recognized in AOCI. In addition, this column includes remaining unrealized gain or loss amounts on securities that were transferred to an HTM designation in the first quarter of 2009 for those securities that are in a net unrealized/unrecognized loss position.
[2] Unrecognized gains represent fair value fluctuations from the later of: (i) the date a security is designated as HTM; or (ii) the date that an OTTI charge is recognized on an HTM security.