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Investments (Components Of Net Realized Gains, Excluding OTTI Charges) (Details) (USD $) | 3 Months Ended
--- | ---
Mar. 31, 2015 | Mar. 31, 2014
Schedule of Available-for-sale Securities [Line Items] |  |
HTM fixed income securities Gains | $ 0 sigi_HeldToMaturitySecuritiesRealizedGain | $ 0 sigi_HeldToMaturitySecuritiesRealizedGain
HTM fixed income securities Losses | (1,000)sigi_HeldToMaturitySecuritiesDisposalSecurityRealizedLoss | (11,000)sigi_HeldToMaturitySecuritiesDisposalSecurityRealizedLoss
Other investments Gains | 0 sigi_OtherInvestmentsRealizedGains | 0 sigi_OtherInvestmentsRealizedGains
Other Investments Losses | (654,000)sigi_OtherInvestmentsRealizedLosses | 0 sigi_OtherInvestmentsRealizedLosses
Total other net realized investment gains | 20,977,000 us-gaap_GainLossOnSaleOfSecuritiesNet | 8,181,000 us-gaap_GainLossOnSaleOfSecuritiesNet
Proceeds From Sale Of Available For Sale Securities | 138,400,000 us-gaap_ProceedsFromSaleOfAvailableForSaleSecurities | 62,800,000 us-gaap_ProceedsFromSaleOfAvailableForSaleSecurities
Fixed Income Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
AFS securities Gains | 1,502,000 us-gaap_AvailableForSaleSecuritiesGrossRealizedGains / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember | 158,000 us-gaap_AvailableForSaleSecuritiesGrossRealizedGains / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember
AFS securities Losses | (112,000)us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember | (112,000)us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember
Equity Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
AFS securities Gains | 21,318,000 us-gaap_AvailableForSaleSecuritiesGrossRealizedGains / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 8,317,000 us-gaap_AvailableForSaleSecuritiesGrossRealizedGains / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
AFS securities Losses | $ (1,076,000)us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | $ (171,000)us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember

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