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Investments (Available-For-Sale Securities Disclosure) (Details) (USD $) | Mar. 31, 2015 | Dec. 31, 2014
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Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | $ 4,258,651,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost |  | $ 4,134,797,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost |
Unrealized Gains | 133,919,000 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax |  | 133,303,000 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax |
Unrealized Losses | (6,757,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax |  | (10,578,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax |
Total AFS securities | 4,385,813,000 us-gaap_AvailableForSaleSecurities |  | 4,257,522,000 us-gaap_AvailableForSaleSecurities |
AFS Fixed Income Securities [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | 4,059,034,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember |  | 3,975,786,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember |
Unrealized Gains | 118,696,000 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember |  | 100,578,000 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember |
Unrealized Losses | (3,488,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember |  | (10,242,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember |
Total AFS securities | 4,174,242,000 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember |  | 4,066,122,000 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember |
AFS Fixed Income Securities [Member] | U.S. Government and Government Agencies [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | 112,146,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember |  | 116,666,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember |
Unrealized Gains | 7,342,000 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember |  | 7,592,000 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember |
Unrealized Losses | (2,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember |  | (128,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember |
Total AFS securities | 119,486,000 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember |  | 124,130,000 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember |
AFS Fixed Income Securities [Member] | Foreign Government [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | 27,030,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember |  | 27,035,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember |
Unrealized Gains | 885,000 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember |  | 796,000 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember |
Unrealized Losses | 0 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember |  | 0 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember |
Total AFS securities | 27,915,000 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember |  | 27,831,000 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember |
AFS Fixed Income Securities [Member] | Obligations of States and Political Subdivisions [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | 1,254,037,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |  | 1,208,776,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |
Unrealized Gains | 40,872,000 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |  | 38,217,000 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |
Unrealized Losses | (534,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |  | (729,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |
Total AFS securities | 1,294,375,000 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |  | 1,246,264,000 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |
AFS Fixed Income Securities [Member] | Corporate Securities [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | 1,770,747,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  | 1,763,427,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |
Unrealized Gains | 55,057,000 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  | 42,188,000 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |
Unrealized Losses | (1,539,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  | (5,809,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |
Total AFS securities | 1,824,265,000 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  | 1,799,806,000 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |
AFS Fixed Income Securities [Member] | Asset-Backed Securities [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | 184,278,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |  | 176,837,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |
Unrealized Gains | 1,214,000 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |  | 760,000 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |
Unrealized Losses | (105,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |  | (373,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |
Total AFS securities | 185,387,000 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |  | 177,224,000 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |
AFS Fixed Income Securities [Member] | Commercial Mortgage Backed Securities [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | 197,304,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | [1] | 177,932,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | [1]
Unrealized Gains | 3,335,000 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | [1] | 2,438,000 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | [1]
Unrealized Losses | (128,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | [1] | (777,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | [1]
Total AFS securities | 200,511,000 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | [1] | 179,593,000 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | [1]
Government guaranteed agency securities fair value | 12,900,000 sigi_GovernmentGuaranteedAgencySecuritiesFairValue / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember |  | 13,200,000 sigi_GovernmentGuaranteedAgencySecuritiesFairValue / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember |
AFS Fixed Income Securities [Member] | Residential Mortgage Backed Securities [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | 513,492,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember | [2] | 505,113,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember | [2]
Unrealized Gains | 9,991,000 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember | [2] | 8,587,000 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember | [2]
Unrealized Losses | (1,180,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember | [2] | (2,426,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember | [2]
Total AFS securities | 522,303,000 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember | [2] | 511,274,000 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember | [2]
Government guaranteed agency securities fair value | 29,200,000 sigi_GovernmentGuaranteedAgencySecuritiesFairValue / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember |  | 32,400,000 sigi_GovernmentGuaranteedAgencySecuritiesFairValue / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember |
Equity Securities [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | 199,617,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember |  | 159,011,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember |
Unrealized Gains | 15,223,000 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember |  | 32,725,000 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember |
Unrealized Losses | (3,269,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember |  | (336,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember |
Total AFS securities | $ 211,571,000 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember |  | $ 191,400,000 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember |
[1] | CMBS includes government guaranteed agency securities with a fair value of $12.9 million at March 31, 2015 and $13.2 million at December 31, 2014.
[2] | RMBS includes government guaranteed agency securities with a fair value of $29.2 million at March 31, 2015 and $32.4 million at December 31, 2014.

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