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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 | Mar. 31, 2015 | Dec. 31, 2014
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Carrying Value | $ 95,020 us-gaap_OtherInvestments | $ 99,203 us-gaap_OtherInvestments
Other Investments [Member] |  |
Carrying Value | 95,020 us-gaap_OtherInvestments / us-gaap_InvestmentTypeAxis = us-gaap_OtherInvestmentsMember | 99,203 us-gaap_OtherInvestments / us-gaap_InvestmentTypeAxis = us-gaap_OtherInvestmentsMember
Remaining Commitment | 66,771 sigi_RemainingCommitment / us-gaap_InvestmentTypeAxis = us-gaap_OtherInvestmentsMember |
Alternative Investments [Member] |  |
Carrying Value | 84,274 us-gaap_OtherInvestments / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember | 92,968 us-gaap_OtherInvestments / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Remaining Commitment | 64,770 sigi_RemainingCommitment / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember |
Secondary Private Equity Member | Alternative Investments [Member] |  |
Carrying Value | 20,208 us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_SecondaryPrivateEquityMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember | 21,807 us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_SecondaryPrivateEquityMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Remaining Commitment | 7,040 sigi_RemainingCommitment / invest_InvestmentSectorAxis = sigi_SecondaryPrivateEquityMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember |
Private Equity [Member] | Alternative Investments [Member] |  |
Carrying Value | 17,519 us-gaap_OtherInvestments / invest_InvestmentSectorAxis = us-gaap_PrivateEquityFundsMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember | 20,126 us-gaap_OtherInvestments / invest_InvestmentSectorAxis = us-gaap_PrivateEquityFundsMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Remaining Commitment | 8,879 sigi_RemainingCommitment / invest_InvestmentSectorAxis = us-gaap_PrivateEquityFundsMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember |
Energy Power Generation [Member] | Alternative Investments [Member] |  |
Carrying Value | 12,393 us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_EnergyPowerGenerationMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember | 14,445 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 Strategy [Member] | Alternative Investments [Member] |  |
Carrying Value | 11,986 us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_RealEstateStrategyMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember | 11,452 us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_RealEstateStrategyMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Remaining Commitment | 10,016 sigi_RemainingCommitment / invest_InvestmentSectorAxis = sigi_RealEstateStrategyMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember |
Mezzanine Financing [Member] | Alternative Investments [Member] |  |
Carrying Value | 8,363 us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_MezzanineFinancingMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember | 9,853 us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_MezzanineFinancingMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Remaining Commitment | 13,598 sigi_RemainingCommitment / invest_InvestmentSectorAxis = sigi_MezzanineFinancingMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember |
Distressed debt [Member] | Alternative Investments [Member] |  |
Carrying Value | 7,569 us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_DistressedDebtMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember | 8,679 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 [Member] | Alternative Investments [Member] |  |
Carrying Value | 6,236 us-gaap_OtherInvestments / invest_InvestmentSectorAxis = us-gaap_VentureCapitalFundsMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember | 6,606 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 | 10,746 us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_OtherSecuritiesMember | 6,235 us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_OtherSecuritiesMember
Remaining Commitment | $ 2,001 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 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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