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Retirement Plans (Schedule of Alternative Investment Portfolio by Strategy and Remaining Commitment) (Details) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Carrying Value | $ 99,203 us-gaap_OtherInvestments | $ 107,875 us-gaap_OtherInvestments
Alternative Investments [Member] |  |
Carrying Value | 92,968 us-gaap_OtherInvestments / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember | 106,211 us-gaap_OtherInvestments / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Remaining Commitment | 64,720 sigi_RemainingCommitment / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember |
Alternative Investments [Member] | Private Equity [Member] |  |
Carrying Value | 20,126 us-gaap_OtherInvestments / invest_InvestmentSectorAxis = us-gaap_PrivateEquityFundsMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember | 20,192 us-gaap_OtherInvestments / invest_InvestmentSectorAxis = us-gaap_PrivateEquityFundsMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Remaining Commitment | 8,890 sigi_RemainingCommitment / invest_InvestmentSectorAxis = us-gaap_PrivateEquityFundsMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember |
Alternative Investments [Member] | Real Estate Strategy [Member] |  |
Carrying Value | 11,452 us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_RealEstateStrategyMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember | 11,698 us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_RealEstateStrategyMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Remaining Commitment | 10,051 sigi_RemainingCommitment / invest_InvestmentSectorAxis = sigi_RealEstateStrategyMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember |
Retirement Income Plan [Member] | Alternative Investments [Member] |  |
Carrying Value | 10,392 us-gaap_OtherInvestments / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember | 12,159 us-gaap_OtherInvestments / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Remaining Commitment | 3,555 sigi_RemainingCommitment / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember |
Retirement Income Plan [Member] | Alternative Investments [Member] | Equity Long Short Hedge [Member] |  |
Carrying Value | 41 us-gaap_OtherInvestments / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / invest_InvestmentSectorAxis = us-gaap_HedgeFundsEquityLongShortMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember | 41 us-gaap_OtherInvestments / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / invest_InvestmentSectorAxis = us-gaap_HedgeFundsEquityLongShortMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Remaining Commitment | 0 sigi_RemainingCommitment / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / invest_InvestmentSectorAxis = us-gaap_HedgeFundsEquityLongShortMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember |
Retirement Income Plan [Member] | Alternative Investments [Member] | Private Equity [Member] |  |
Carrying Value | 8,136 us-gaap_OtherInvestments / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / invest_InvestmentSectorAxis = us-gaap_PrivateEquityFundsMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember | 9,899 us-gaap_OtherInvestments / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / invest_InvestmentSectorAxis = us-gaap_PrivateEquityFundsMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Remaining Commitment | 3,019 sigi_RemainingCommitment / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / invest_InvestmentSectorAxis = us-gaap_PrivateEquityFundsMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember |
Retirement Income Plan [Member] | Alternative Investments [Member] | Real Estate Strategy [Member] |  |
Carrying Value | 2,215 us-gaap_OtherInvestments / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / invest_InvestmentSectorAxis = sigi_RealEstateStrategyMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember | 2,219 us-gaap_OtherInvestments / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / invest_InvestmentSectorAxis = sigi_RealEstateStrategyMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Remaining Commitment | $ 536 sigi_RemainingCommitment / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / invest_InvestmentSectorAxis = sigi_RealEstateStrategyMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember |

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