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Investments (Components Of Net Realized Gains, Excluding OTTI Charges) (Details) (USD $) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013 | Mar. 31, 2013 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
HTM fixed income securities gains |  |  |  |  |  |  |  |  | $ 2,000 sigi_HeldToMaturitySecuritiesRealizedGain | $ 195,000 sigi_HeldToMaturitySecuritiesRealizedGain | $ 194,000 sigi_HeldToMaturitySecuritiesRealizedGain
HTM fixed income securities losses |  |  |  |  |  |  |  |  | (20,000)sigi_HeldToMaturitySecuritiesSoldSecurityRealizedLoss | (95,000)sigi_HeldToMaturitySecuritiesSoldSecurityRealizedLoss | (217,000)sigi_HeldToMaturitySecuritiesSoldSecurityRealizedLoss
Short term investments realized losses |  |  |  |  |  |  |  |  | 0 sigi_ShortTermInvestmentsRealizedLosses | 0 sigi_ShortTermInvestmentsRealizedLosses | (2,000)sigi_ShortTermInvestmentsRealizedLosses
Other investment securities gains |  |  |  |  |  |  |  |  | 1,000 sigi_OtherInvestmentsRealizedGains | 0 sigi_OtherInvestmentsRealizedGains | 1,000 sigi_OtherInvestmentsRealizedGains
Other investment securities losses |  |  |  |  |  |  |  |  | 0 sigi_OtherInvestmentsRealizedLosses | (1,060,000)sigi_OtherInvestmentsRealizedLosses | (400,000)sigi_OtherInvestmentsRealizedLosses
Net realized investment gains | (389,000)us-gaap_GainLossOnSaleOfSecuritiesNet | 15,231,000 us-gaap_GainLossOnSaleOfSecuritiesNet | 4,539,000 us-gaap_GainLossOnSaleOfSecuritiesNet | 7,218,000 us-gaap_GainLossOnSaleOfSecuritiesNet | (1,208,000)us-gaap_GainLossOnSaleOfSecuritiesNet | 13,431,000 us-gaap_GainLossOnSaleOfSecuritiesNet | 5,154,000 us-gaap_GainLossOnSaleOfSecuritiesNet | 3,355,000 us-gaap_GainLossOnSaleOfSecuritiesNet | 37,703,000 us-gaap_GainLossOnSaleOfSecuritiesNet | 26,375,000 us-gaap_GainLossOnSaleOfSecuritiesNet | 13,252,000 us-gaap_GainLossOnSaleOfSecuritiesNet
Proceeds from sale of available-for-sale securities |  |  |  |  |  |  |  |  | 259,000,000 us-gaap_ProceedsFromSaleOfAvailableForSaleSecurities | 135,900,000 us-gaap_ProceedsFromSaleOfAvailableForSaleSecurities | 205,300,000 us-gaap_ProceedsFromSaleOfAvailableForSaleSecurities
Realized Gains On Equity Portfolio Related To Change In Strategy [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
AFS securities gains |  |  |  |  |  |  |  |  | 9,200,000 us-gaap_AvailableForSaleSecuritiesGrossRealizedGains / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = sigi_RealizedGainsOnEquityPortfolioRelatedToChangeInStrategyMember |  |
Fixed Income Securities [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
AFS securities gains |  |  |  |  |  |  |  |  | 1,945,000 us-gaap_AvailableForSaleSecuritiesGrossRealizedGains / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember | 3,340,000 us-gaap_AvailableForSaleSecuritiesGrossRealizedGains / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember | 4,452,000 us-gaap_AvailableForSaleSecuritiesGrossRealizedGains / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember
AFS securities losses |  |  |  |  |  |  |  |  | (392,000)us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember | (373,000)us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember | (472,000)us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember
Equity Securities [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
AFS securities gains |  |  |  |  |  |  |  |  | 36,871,000 us-gaap_AvailableForSaleSecuritiesGrossRealizedGains / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 24,776,000 us-gaap_AvailableForSaleSecuritiesGrossRealizedGains / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 10,901,000 us-gaap_AvailableForSaleSecuritiesGrossRealizedGains / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
AFS securities losses |  |  |  |  |  |  |  |  | $ (704,000)us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | $ (408,000)us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | $ (1,205,000)us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember

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