XML 148 R58.htm IDEA: XBRL DOCUMENT
v2.4.1.9

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 | $ 4,134,797,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost |  | $ 3,831,327,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost |
Unrealized Gains | 133,299,000 sigi_AFSSecuritiesUnrealizedGainGross |  | 128,951,000 sigi_AFSSecuritiesUnrealizedGainGross |
Unrealized Losses | (10,574,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax |  | (51,971,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax |
Total AFS securities | 4,257,522,000 us-gaap_AvailableForSaleSecurities |  | 3,908,307,000 us-gaap_AvailableForSaleSecurities |
AFS Fixed Income Securities [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | 3,975,786,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember |  | 3,675,977,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember |
Unrealized Gains | 100,578,000 sigi_AFSSecuritiesUnrealizedGainGross / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember |  | 91,434,000 sigi_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 | 4,066,122,000 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember |  | 3,715,536,000 us-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 | 116,666,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember |  | 163,218,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember |
Unrealized Gains | 7,592,000 sigi_AFSSecuritiesUnrealizedGainGross / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember |  | 10,661,000 sigi_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 | 124,130,000 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember |  | 173,375,000 us-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 | 27,035,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember |  | 29,781,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember |
Unrealized Gains | 796,000 sigi_AFSSecuritiesUnrealizedGainGross / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember |  | 906,000 sigi_AFSSecuritiesUnrealizedGainGross / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember |
Unrealized Losses | 0 us-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 | 27,831,000 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember |  | 30,615,000 us-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 | 1,208,776,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |  | 946,455,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |
Unrealized Gains | 38,217,000 sigi_AFSSecuritiesUnrealizedGainGross / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |  | 25,194,000 sigi_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 | 1,246,264,000 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |  | 951,624,000 us-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 | 1,763,427,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  | 1,707,928,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |
Unrealized Gains | 42,188,000 sigi_AFSSecuritiesUnrealizedGainGross / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  | 44,004,000 sigi_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 | 1,799,806,000 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  | 1,734,883,000 us-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 | 176,837,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |  | 140,430,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |
Unrealized Gains | 760,000 sigi_AFSSecuritiesUnrealizedGainGross / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |  | 934,000 sigi_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 | 177,224,000 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |  | 140,896,000 us-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 | 177,932,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | [1] | 172,288,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | [1]
Unrealized Gains | 2,438,000 sigi_AFSSecuritiesUnrealizedGainGross / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | [1] | 2,462,000 sigi_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 | 179,593,000 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | [1] | 171,284,000 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | [1]
Government guaranteed agency securities fair value | 13,200,000 sigi_GovernmentGuaranteedAgencySecuritiesFairValue / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember |  | 30,000,000 sigi_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 | 505,113,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember | [2] | 515,877,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember | [2]
Unrealized Gains | 8,587,000 sigi_AFSSecuritiesUnrealizedGainGross / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember | [2] | 7,273,000 sigi_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 | 511,274,000 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember | [2] | 512,859,000 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember | [2]
Government guaranteed agency securities fair value | 32,400,000 sigi_GovernmentGuaranteedAgencySecuritiesFairValue / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember |  | 55,200,000 sigi_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 | 159,011,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember |  | 155,350,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember |
Unrealized Gains | 32,721,000 sigi_AFSSecuritiesUnrealizedGainGross / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember |  | 37,517,000 sigi_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 | $ 191,400,000 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember |  | $ 192,771,000 us-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.

X
- Definition Amount of unrealized gain before deducting unrealized loss on investments in debt and equity securities classified as available-for-sale securities. + References No definition available. + Details
Name: | sigi_AFSSecuritiesUnrealizedGainGross
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The fair value of agency securities that are issued by a particular agency of the federal government which are guaranteed by the U.S. government. + References No definition available. + Details
Name: | sigi_GovernmentGuaranteedAgencySecuritiesFairValue
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562 + Details
Name: | us-gaap_AvailableForSaleSecurities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563 + Details
Name: | us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_AvailableForSaleSecuritiesAmortizedCost
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_InvestmentTypeAxis=sigi_AfsFixedMaturitySecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InvestmentTypeAxis=us-gaap_EquitySecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |