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v2.4.1.9

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
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AFS Fair Value, Less than 12 months | $ 371,381 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue |  | $ 466,387 us-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 | 156,812 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue |  | 442,058 us-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 | 197 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue |  | 196 us-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 | 1 sigi_HeldToMaturitySecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionLessThanTwelveMonths | [2] | 1 sigi_HeldToMaturitySecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionLessThanTwelveMonths | [2]
HTM Fair Value, 12 months or longer | 2,003 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue |  | 2,235 us-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 | 361 sigi_HeldToMaturitySecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionTwelveMonthsorLonger | [2] | 439 sigi_HeldToMaturitySecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionTwelveMonthsorLonger | [2]
Total AFS and HTM Fair Value, Less than 12 months | 371,578 sigi_AvailableForSaleandHeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue |  | 466,583 sigi_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 | 1 sigi_HeldToMaturityAndAvailableForSaleSecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionLessThanTwelveMonths | [2] | 1 sigi_HeldToMaturityAndAvailableForSaleSecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionLessThanTwelveMonths | [2]
Total AFS and HTM Fair Value, 12 months or longer | 158,815 sigi_AvailableForSaleandHeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue |  | 444,293 sigi_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 | 361 sigi_HeldToMaturityAndAvailableForSaleSecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionForTwelveMonthsOrLonger | [2] | 439 sigi_HeldToMaturityAndAvailableForSaleSecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionForTwelveMonthsOrLonger | [2]
Obligations of States and Political Subdivisions [Member] |  |  |  |
HTM Fair Value, Less than 12 months | 197 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |  | 196 us-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 | 1 sigi_HeldToMaturitySecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionLessThanTwelveMonths / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | [2] | 1 sigi_HeldToMaturitySecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionLessThanTwelveMonths / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | [2]
HTM Fair Value, 12 months or longer | 0 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |  | 0 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |
HTM Unrealized Losses, 12 months or longer | 0 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | [1] | 0 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | [1]
HTM Unrecognized Gains, 12 months or longer | 0 sigi_HeldToMaturitySecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionTwelveMonthsorLonger / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | [2] | 0 sigi_HeldToMaturitySecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionTwelveMonthsorLonger / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | [2]
Asset-Backed Securities [Member] |  |  |  |
HTM Fair Value, Less than 12 months | 0 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember |  | 0 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember |
HTM Unrealized Losses, Less than 12 months | 0 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember | [1] | 0 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember | [1]
HTM Unrecognized Gains, Less than 12 months | 0 sigi_HeldToMaturitySecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionLessThanTwelveMonths / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember | [2] | 0 sigi_HeldToMaturitySecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionLessThanTwelveMonths / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember | [2]
HTM Fair Value, 12 months or longer | 2,003 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember |  | 2,235 us-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 | 361 sigi_HeldToMaturitySecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionTwelveMonthsorLonger / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember | [2] | 439 sigi_HeldToMaturitySecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionTwelveMonthsorLonger / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember | [2]
AFS Fixed Income Securities [Member] |  |  |  |
AFS Fair Value, Less than 12 months | 271,172 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AFSFixedIncomeSecuritiesMember |  | 461,125 us-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 | 156,812 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AFSFixedIncomeSecuritiesMember |  | 442,058 us-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 | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_InvestmentTypeAxis = us-gaap_USTreasuryAndGovernmentMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AFSFixedIncomeSecuritiesMember |  | 7,567 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_InvestmentTypeAxis = us-gaap_USTreasuryAndGovernmentMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AFSFixedIncomeSecuritiesMember |
AFS Unrealized Losses, Less than 12 months | 0 us-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 | 398 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_InvestmentTypeAxis = us-gaap_USTreasuryAndGovernmentMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AFSFixedIncomeSecuritiesMember |  | 10,866 us-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 | 75,935 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AFSFixedIncomeSecuritiesMember |  | 47,510 us-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 | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AFSFixedIncomeSecuritiesMember |  | 64,018 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AFSFixedIncomeSecuritiesMember |
AFS Unrealized Losses, 12 months or longer | 0 us-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 | 93,912 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_InvestmentTypeAxis = us-gaap_CorporateDebtSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AFSFixedIncomeSecuritiesMember |  | 276,648 us-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 | 50,858 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_InvestmentTypeAxis = us-gaap_CorporateDebtSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AFSFixedIncomeSecuritiesMember |  | 153,613 us-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 | 25,684 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AFSFixedIncomeSecuritiesMember |  | 113,202 us-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 | 14,353 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AFSFixedIncomeSecuritiesMember |  | 15,618 us-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 | 24,664 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_InvestmentTypeAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AFSFixedIncomeSecuritiesMember |  | 12,799 us-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 | 15,682 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_InvestmentTypeAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AFSFixedIncomeSecuritiesMember |  | 59,219 us-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 | 50,977 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_InvestmentTypeAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AFSFixedIncomeSecuritiesMember |  | 3,399 us-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 | 75,521 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_InvestmentTypeAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AFSFixedIncomeSecuritiesMember |  | 138,724 us-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 | 100,209 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = us-gaap_EquitySecuritiesMember |  | 5,262 us-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 | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = us-gaap_EquitySecuritiesMember |  | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = us-gaap_EquitySecuritiesMember |
AFS Unrealized Losses, 12 months or longer | $ 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = us-gaap_EquitySecuritiesMember | [1] | $ 0 us-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.

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- Definition This item represents the unrealized holding gains/losses of an available-for sale ("AFS") and held-to-maturity("HTM") security that has been in a continuous unrealized/unrecognized loss position for less than twelve months. The AFS unrealized holding losses represent the excess of amortized cost over fair value of securities. The HTM unrealized holding gains/losses represent the remaining unrealized gain/loss amounts on securities that were transferred to a HTM designation from an AFS designation as well as other-than-temporary impairment losses that are recognized in accumulated other comprehensive income. + References No definition available. + Details
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- Definition This item represents the unrealized holding gains/losses of an available-for sale ("AFS") and held-to-maturity("HTM") security that has been in a continuous unrealized/unrecognized loss position for twelve months or longer. The AFS unrealized holding losses represent the excess of amortized cost over fair value of securities. The HTM unrealized holding gains/losses represent the remaining unrealized gain/loss amounts on securities that were transferred to a HTM designation from an AFS designation as well as other-than-temporary impairment losses that are recognized in accumulated other comprehensive income. + References No definition available. + Details
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- Definition This item represents the unrecognized holding gains/losses of a security that has been in a continuous unrealized/unrecognized loss position for twelve months or longer. The unrecognized holding gains/losses represent the market value fluctuations from the later of: (i) the date a security is designated as held-to-maturity; or (ii) the date that an other-than-temporary- impairment charge is recognized on a held-to-maturity security. + References No definition available. + Details
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- Definition This item represents the unrecognized holding gains/losses of a security that has been in a continuous unrealized/unrecognized loss position for less than twelve months. The unrecognized holding gains/losses represent the market value fluctuations from the later of: (i) the date a security is designated as held-to-maturity; or (ii) the date that an other-than-temporary- impairment charge is recognized on a held-to-maturity security. + References No definition available. + Details
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- Definition This item represents the unrecognized holding gains/losses of a held-to-maturity security that has been in a continuous unrealized/unrecognized loss position for twelve months or longer. The unrecognized holding gains/losses represent the market value fluctuations from the later of: (i) the date a security is designated as held-to-maturity; or (ii) the date that an other-than-temporary- impairment charge is recognized on a held-to-maturity security. + References No definition available. + Details
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