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v2.4.1.9

Investments (Held-To-Maturity Securities Disclosure) (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended |
--- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013
Amortized Cost | $ 317,179 us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment | $ 390,622 us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
Net Unrealized Gains (Losses) | 958 sigi_HeldToMaturitySecuritiesNetUnrealizedGainsLosses | 2,257 sigi_HeldToMaturitySecuritiesNetUnrealizedGainsLosses
Total HTM fixed income securities carrying value | 318,137 us-gaap_HeldToMaturitySecurities | 392,879 us-gaap_HeldToMaturitySecurities
Unrecognized Holding Gains | 15,824 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain | 24,102 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
Unrecognized Holding Losses | 0 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss | 0 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
Total HTM fixed income securities fair value | 333,961 us-gaap_HeldToMaturitySecuritiesFairValue | 416,981 us-gaap_HeldToMaturitySecuritiesFairValue
Held To Maturity Securities Average Duration | 1 year 8 months 8 days |
Foreign Government [Member] |  |
Amortized Cost | 5,292 us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment / us-gaap_DebtSecurityAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember | 5,292 us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment / us-gaap_DebtSecurityAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember
Net Unrealized Gains (Losses) | 47 sigi_HeldToMaturitySecuritiesNetUnrealizedGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember | 131 sigi_HeldToMaturitySecuritiesNetUnrealizedGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember
Total HTM fixed income securities carrying value | 5,339 us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember | 5,423 us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember
Unrecognized Holding Gains | 55 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_DebtSecurityAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember | 168 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_DebtSecurityAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember
Unrecognized Holding Losses | 0 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss / us-gaap_DebtSecurityAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember | 0 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss / us-gaap_DebtSecurityAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember
Total HTM fixed income securities fair value | 5,394 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember | 5,591 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember
Obligations of States and Political Subdivisions [Member] |  |
Amortized Cost | 285,301 us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 348,109 us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Net Unrealized Gains (Losses) | 2,071 sigi_HeldToMaturitySecuritiesNetUnrealizedGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 4,013 sigi_HeldToMaturitySecuritiesNetUnrealizedGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Total HTM fixed income securities carrying value | 287,372 us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 352,122 us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Unrecognized Holding Gains | 11,760 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 17,634 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Unrecognized Holding Losses | 0 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 0 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Total HTM fixed income securities fair value | 299,132 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 369,756 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Corporate Securities [Member] |  |
Amortized Cost | 18,899 us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment / us-gaap_DebtSecurityAxis = us-gaap_CorporateDebtSecuritiesMember | 28,174 us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment / us-gaap_DebtSecurityAxis = us-gaap_CorporateDebtSecuritiesMember
Net Unrealized Gains (Losses) | (273)sigi_HeldToMaturitySecuritiesNetUnrealizedGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_CorporateDebtSecuritiesMember | (346)sigi_HeldToMaturitySecuritiesNetUnrealizedGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_CorporateDebtSecuritiesMember
Total HTM fixed income securities carrying value | 18,626 us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = us-gaap_CorporateDebtSecuritiesMember | 27,828 us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = us-gaap_CorporateDebtSecuritiesMember
Unrecognized Holding Gains | 2,796 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_DebtSecurityAxis = us-gaap_CorporateDebtSecuritiesMember | 2,446 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_DebtSecurityAxis = us-gaap_CorporateDebtSecuritiesMember
Unrecognized Holding Losses | 0 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss / us-gaap_DebtSecurityAxis = us-gaap_CorporateDebtSecuritiesMember | 0 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss / us-gaap_DebtSecurityAxis = us-gaap_CorporateDebtSecuritiesMember
Total HTM fixed income securities fair value | 21,422 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_CorporateDebtSecuritiesMember | 30,274 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_CorporateDebtSecuritiesMember
Asset-backed Securities [Member] |  |
Amortized Cost | 2,818 us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment / us-gaap_DebtSecurityAxis = us-gaap_AssetBackedSecuritiesMember | 3,413 us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment / us-gaap_DebtSecurityAxis = us-gaap_AssetBackedSecuritiesMember
Net Unrealized Gains (Losses) | (455)sigi_HeldToMaturitySecuritiesNetUnrealizedGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_AssetBackedSecuritiesMember | (655)sigi_HeldToMaturitySecuritiesNetUnrealizedGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_AssetBackedSecuritiesMember
Total HTM fixed income securities carrying value | 2,363 us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = us-gaap_AssetBackedSecuritiesMember | 2,758 us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = us-gaap_AssetBackedSecuritiesMember
Unrecognized Holding Gains | 460 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_DebtSecurityAxis = us-gaap_AssetBackedSecuritiesMember | 657 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_DebtSecurityAxis = us-gaap_AssetBackedSecuritiesMember
Unrecognized Holding Losses | 0 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss / us-gaap_DebtSecurityAxis = us-gaap_AssetBackedSecuritiesMember | 0 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss / us-gaap_DebtSecurityAxis = us-gaap_AssetBackedSecuritiesMember
Total HTM fixed income securities fair value | 2,823 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_AssetBackedSecuritiesMember | 3,415 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_AssetBackedSecuritiesMember
Commercial Mortgage Backed Securities [Member] |  |
Amortized Cost | 4,869 us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment / us-gaap_DebtSecurityAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | 5,634 us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment / us-gaap_DebtSecurityAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
Net Unrealized Gains (Losses) | (432)sigi_HeldToMaturitySecuritiesNetUnrealizedGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | (886)sigi_HeldToMaturitySecuritiesNetUnrealizedGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
Total HTM fixed income securities carrying value | 4,437 us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | 4,748 us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
Unrecognized Holding Gains | 753 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_DebtSecurityAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | 3,197 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_DebtSecurityAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
Unrecognized Holding Losses | 0 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss / us-gaap_DebtSecurityAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | 0 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss / us-gaap_DebtSecurityAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
Total HTM fixed income securities fair value | $ 5,190 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | $ 7,945 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_CommercialMortgageBackedSecuritiesMember

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