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Investments (Held-To-Maturity Securities Disclosure) (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended |
--- | --- | ---
Mar. 31, 2015 | Dec. 31, 2014
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | $ 295,859 us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment | $ 317,179 us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
Net Unrealized Gains (Losses) | 698 sigi_HeldToMaturitySecuritiesNetUnrealizedGainsLosses | 958 sigi_HeldToMaturitySecuritiesNetUnrealizedGainsLosses
Carrying Value | 296,557 us-gaap_HeldToMaturitySecurities | 318,137 us-gaap_HeldToMaturitySecurities
Unrecognized Holding Gains | 14,536 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain | 15,824 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
Unrecognized Holding Losses | 0 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss | 0 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
HTM, Fair Value | 311,093 us-gaap_HeldToMaturitySecuritiesFairValue | 333,961 us-gaap_HeldToMaturitySecuritiesFairValue
Held To Maturity securities average duration | 1 year 8 months 8 days |
Foreign Government [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
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) | 25 sigi_HeldToMaturitySecuritiesNetUnrealizedGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember | 47 sigi_HeldToMaturitySecuritiesNetUnrealizedGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember
Carrying Value | 5,317 us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember | 5,339 us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember
Unrecognized Holding Gains | 21 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_DebtSecurityAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember | 55 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
HTM, Fair Value | 5,338 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember | 5,394 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember
Obligations of States and Political Subdivisions [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 264,788 us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 285,301 us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Net Unrealized Gains (Losses) | 1,706 sigi_HeldToMaturitySecuritiesNetUnrealizedGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 2,071 sigi_HeldToMaturitySecuritiesNetUnrealizedGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Carrying Value | 266,494 us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 287,372 us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Unrecognized Holding Gains | 10,536 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 11,760 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
HTM, Fair Value | 277,030 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 299,132 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Corporate Securities [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 18,532 us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment / us-gaap_DebtSecurityAxis = us-gaap_CorporateDebtSecuritiesMember | 18,899 us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment / us-gaap_DebtSecurityAxis = us-gaap_CorporateDebtSecuritiesMember
Net Unrealized Gains (Losses) | (269)sigi_HeldToMaturitySecuritiesNetUnrealizedGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_CorporateDebtSecuritiesMember | (273)sigi_HeldToMaturitySecuritiesNetUnrealizedGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_CorporateDebtSecuritiesMember
Carrying Value | 18,263 us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = us-gaap_CorporateDebtSecuritiesMember | 18,626 us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = us-gaap_CorporateDebtSecuritiesMember
Unrecognized Holding Gains | 2,936 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_DebtSecurityAxis = us-gaap_CorporateDebtSecuritiesMember | 2,796 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
HTM, Fair Value | 21,199 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_CorporateDebtSecuritiesMember | 21,422 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_CorporateDebtSecuritiesMember
Asset-Backed Securities [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 2,483 us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment / us-gaap_DebtSecurityAxis = us-gaap_AssetBackedSecuritiesMember | 2,818 us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment / us-gaap_DebtSecurityAxis = us-gaap_AssetBackedSecuritiesMember
Net Unrealized Gains (Losses) | (378)sigi_HeldToMaturitySecuritiesNetUnrealizedGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_AssetBackedSecuritiesMember | (455)sigi_HeldToMaturitySecuritiesNetUnrealizedGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_AssetBackedSecuritiesMember
Carrying Value | 2,105 us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = us-gaap_AssetBackedSecuritiesMember | 2,363 us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = us-gaap_AssetBackedSecuritiesMember
Unrecognized Holding Gains | 375 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_DebtSecurityAxis = us-gaap_AssetBackedSecuritiesMember | 460 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
HTM, Fair Value | 2,480 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_AssetBackedSecuritiesMember | 2,823 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_AssetBackedSecuritiesMember
Commercial Mortgage Backed Securities [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 4,764 us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment / us-gaap_DebtSecurityAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | 4,869 us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment / us-gaap_DebtSecurityAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
Net Unrealized Gains (Losses) | (386)sigi_HeldToMaturitySecuritiesNetUnrealizedGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | (432)sigi_HeldToMaturitySecuritiesNetUnrealizedGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
Carrying Value | 4,378 us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | 4,437 us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
Unrecognized Holding Gains | 668 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_DebtSecurityAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | 753 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
HTM, Fair Value | $ 5,046 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | $ 5,190 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_CommercialMortgageBackedSecuritiesMember

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- Definition The remaining aggregate increase (decrease) in the market value of held-to-maturity securities either (i) prior to the securities' transfer to a held-to-maturity designation or (ii) resulting from a non-credit other-than-temporary impairment. + References No definition available. + Details
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