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Investments (Other Investment Portfolio by Strategy and the Remaining Commitment Amount Associated With Each Strategy) (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 |
Other Investment Portfolio [Member] |  |
Carrying Value | 99,203 us-gaap_OtherInvestments / us-gaap_InvestmentTypeAxis = us-gaap_OtherInvestmentsMember | 107,875 us-gaap_OtherInvestments / us-gaap_InvestmentTypeAxis = us-gaap_OtherInvestmentsMember
Remaining Commitment | 68,431 sigi_RemainingCommitment / us-gaap_InvestmentTypeAxis = us-gaap_OtherInvestmentsMember |
Secondary Private Equity [Member] | Alternative Investments [Member] |  |
Carrying Value | 21,807 us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_SecondaryPrivateEquityMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember | 25,618 us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_SecondaryPrivateEquityMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Remaining Commitment | 7,001 sigi_RemainingCommitment / invest_InvestmentSectorAxis = sigi_SecondaryPrivateEquityMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember |
Private Equity [Member] | Alternative Investments [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 |
Energy Power Generation [Member] | Alternative Investments [Member] |  |
Carrying Value | 14,445 us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_EnergyPowerGenerationMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember | 17,361 us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_EnergyPowerGenerationMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Remaining Commitment | 21,905 sigi_RemainingCommitment / invest_InvestmentSectorAxis = sigi_EnergyPowerGenerationMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember |
Real Estate [Member] | Alternative Investments [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 |
Mezzanine Financing [Member] | Alternative Investments [Member] |  |
Carrying Value | 9,853 us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_MezzanineFinancingMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember | 12,738 us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_MezzanineFinancingMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Remaining Commitment | 13,541 sigi_RemainingCommitment / invest_InvestmentSectorAxis = sigi_MezzanineFinancingMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember |
Distressed Debt [Member] | Alternative Investments [Member] |  |
Carrying Value | 8,679 us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_DistressedDebtMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember | 11,579 us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_DistressedDebtMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Remaining Commitment | 2,982 sigi_RemainingCommitment / invest_InvestmentSectorAxis = sigi_DistressedDebtMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember |
Venture Capital Funds [Member] | Alternative Investments [Member] |  |
Carrying Value | 6,606 us-gaap_OtherInvestments / invest_InvestmentSectorAxis = us-gaap_VentureCapitalFundsMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember | 7,025 us-gaap_OtherInvestments / invest_InvestmentSectorAxis = us-gaap_VentureCapitalFundsMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Remaining Commitment | 350 sigi_RemainingCommitment / invest_InvestmentSectorAxis = us-gaap_VentureCapitalFundsMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember |
Other Securities [Member] |  |
Carrying Value | 6,235 us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_OtherSecuritiesMember | 1,664 us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_OtherSecuritiesMember
Remaining Commitment | $ 3,711 sigi_RemainingCommitment / invest_InvestmentSectorAxis = sigi_OtherSecuritiesMember |

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- Definition Other investments not otherwise specified in the taxonomy. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.12) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 12 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 1 -Subparagraph g -Article 7 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.1(f,g)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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