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Investments (Available-For-Sale Securities Disclosure) (Details) (USD $)
Dec. 31, 2014
Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]    
Cost/Amortized Cost $ 4,134,797,000us-gaap_AvailableForSaleSecuritiesAmortizedCost $ 3,831,327,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
Unrealized Gains 133,299,000sigi_AFSSecuritiesUnrealizedGainGross 128,951,000sigi_AFSSecuritiesUnrealizedGainGross
Unrealized Losses (10,574,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax (51,971,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Total AFS securities 4,257,522,000us-gaap_AvailableForSaleSecurities 3,908,307,000us-gaap_AvailableForSaleSecurities
AFS Fixed Income Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Cost/Amortized Cost 3,975,786,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember		 3,675,977,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
Unrealized Gains 100,578,000sigi_AFSSecuritiesUnrealizedGainGross
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember		 91,434,000sigi_AFSSecuritiesUnrealizedGainGross
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
Unrealized Losses (10,242,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember		 (51,875,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
Total AFS securities 4,066,122,000us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember		 3,715,536,000us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
AFS Fixed Income Securities [Member] | U.S. Government and Government Agencies [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Cost/Amortized Cost 116,666,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember		 163,218,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
Unrealized Gains 7,592,000sigi_AFSSecuritiesUnrealizedGainGross
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember		 10,661,000sigi_AFSSecuritiesUnrealizedGainGross
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
Unrealized Losses (128,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember		 (504,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
Total AFS securities 124,130,000us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember		 173,375,000us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ 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 27,035,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember		 29,781,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
Unrealized Gains 796,000sigi_AFSSecuritiesUnrealizedGainGross
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember		 906,000sigi_AFSSecuritiesUnrealizedGainGross
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
Unrealized Losses 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember		 (72,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
Total AFS securities 27,831,000us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember		 30,615,000us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ 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 1,208,776,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 946,455,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Unrealized Gains 38,217,000sigi_AFSSecuritiesUnrealizedGainGross
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 25,194,000sigi_AFSSecuritiesUnrealizedGainGross
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Unrealized Losses (729,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 (20,025,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Total AFS securities 1,246,264,000us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 951,624,000us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ 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 1,763,427,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		 1,707,928,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Unrealized Gains 42,188,000sigi_AFSSecuritiesUnrealizedGainGross
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		 44,004,000sigi_AFSSecuritiesUnrealizedGainGross
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Unrealized Losses (5,809,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		 (17,049,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Total AFS securities 1,799,806,000us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		 1,734,883,000us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ 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 176,837,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember		 140,430,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
Unrealized Gains 760,000sigi_AFSSecuritiesUnrealizedGainGross
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember		 934,000sigi_AFSSecuritiesUnrealizedGainGross
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
Unrealized Losses (373,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember		 (468,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
Total AFS securities 177,224,000us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember		 140,896,000us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ 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 177,932,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember		 [1] 172,288,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember		 [1]
Unrealized Gains 2,438,000sigi_AFSSecuritiesUnrealizedGainGross
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember		 [1] 2,462,000sigi_AFSSecuritiesUnrealizedGainGross
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember		 [1]
Unrealized Losses (777,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember		 [1] (3,466,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember		 [1]
Total AFS securities 179,593,000us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember		 [1] 171,284,000us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember		 [1]
Government guaranteed agency securities fair value 13,200,000sigi_GovernmentGuaranteedAgencySecuritiesFairValue
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember		 30,000,000sigi_GovernmentGuaranteedAgencySecuritiesFairValue
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ 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 505,113,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		 [2] 515,877,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		 [2]
Unrealized Gains 8,587,000sigi_AFSSecuritiesUnrealizedGainGross
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		 [2] 7,273,000sigi_AFSSecuritiesUnrealizedGainGross
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		 [2]
Unrealized Losses (2,426,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		 [2] (10,291,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		 [2]
Total AFS securities 511,274,000us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		 [2] 512,859,000us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		 [2]
Government guaranteed agency securities fair value 32,400,000sigi_GovernmentGuaranteedAgencySecuritiesFairValue
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		 55,200,000sigi_GovernmentGuaranteedAgencySecuritiesFairValue
/ us-gaap_InvestmentTypeAxis
= sigi_AfsFixedMaturitySecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
Equity Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Cost/Amortized Cost 159,011,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= us-gaap_EquitySecuritiesMember		 155,350,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= us-gaap_EquitySecuritiesMember
Unrealized Gains 32,721,000sigi_AFSSecuritiesUnrealizedGainGross
/ us-gaap_InvestmentTypeAxis
= us-gaap_EquitySecuritiesMember		 37,517,000sigi_AFSSecuritiesUnrealizedGainGross
/ us-gaap_InvestmentTypeAxis
= us-gaap_EquitySecuritiesMember
Unrealized Losses (332,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= us-gaap_EquitySecuritiesMember		 (96,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= us-gaap_EquitySecuritiesMember
Total AFS securities $ 191,400,000us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_EquitySecuritiesMember		 $ 192,771,000us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_EquitySecuritiesMember
[1] CMBS includes government guaranteed agency securities with a fair value of $13.2 million at December 31, 2014 and $30.0 million at December 31, 2013
[2] RMBS includes government guaranteed agency securities with a fair value of $32.4 million at December 31, 2014 and $55.2 million at December 31, 2013.