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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
Carrying Value	$ 95,020us-gaap_OtherInvestments	$ 99,203us-gaap_OtherInvestments
Other Investments [Member]
Carrying Value	95,020us-gaap_OtherInvestments / us-gaap_InvestmentTypeAxis = us-gaap_OtherInvestmentsMember	99,203us-gaap_OtherInvestments / us-gaap_InvestmentTypeAxis = us-gaap_OtherInvestmentsMember
Remaining Commitment	66,771sigi_RemainingCommitment / us-gaap_InvestmentTypeAxis = us-gaap_OtherInvestmentsMember
Alternative Investments [Member]
Carrying Value	84,274us-gaap_OtherInvestments / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember	92,968us-gaap_OtherInvestments / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Remaining Commitment	64,770sigi_RemainingCommitment / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Secondary Private Equity Member | Alternative Investments [Member]
Carrying Value	20,208us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_SecondaryPrivateEquityMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember	21,807us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_SecondaryPrivateEquityMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Remaining Commitment	7,040sigi_RemainingCommitment / invest_InvestmentSectorAxis = sigi_SecondaryPrivateEquityMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Private Equity [Member] | Alternative Investments [Member]
Carrying Value	17,519us-gaap_OtherInvestments / invest_InvestmentSectorAxis = us-gaap_PrivateEquityFundsMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember	20,126us-gaap_OtherInvestments / invest_InvestmentSectorAxis = us-gaap_PrivateEquityFundsMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Remaining Commitment	8,879sigi_RemainingCommitment / invest_InvestmentSectorAxis = us-gaap_PrivateEquityFundsMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Energy Power Generation [Member] | Alternative Investments [Member]
Carrying Value	12,393us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_EnergyPowerGenerationMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember	14,445us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_EnergyPowerGenerationMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Remaining Commitment	21,905sigi_RemainingCommitment / invest_InvestmentSectorAxis = sigi_EnergyPowerGenerationMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Real Estate Strategy [Member] | Alternative Investments [Member]
Carrying Value	11,986us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_RealEstateStrategyMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember	11,452us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_RealEstateStrategyMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Remaining Commitment	10,016sigi_RemainingCommitment / invest_InvestmentSectorAxis = sigi_RealEstateStrategyMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Mezzanine Financing [Member] | Alternative Investments [Member]
Carrying Value	8,363us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_MezzanineFinancingMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember	9,853us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_MezzanineFinancingMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Remaining Commitment	13,598sigi_RemainingCommitment / invest_InvestmentSectorAxis = sigi_MezzanineFinancingMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Distressed debt [Member] | Alternative Investments [Member]
Carrying Value	7,569us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_DistressedDebtMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember	8,679us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_DistressedDebtMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Remaining Commitment	2,982sigi_RemainingCommitment / invest_InvestmentSectorAxis = sigi_DistressedDebtMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Venture Capital [Member] | Alternative Investments [Member]
Carrying Value	6,236us-gaap_OtherInvestments / invest_InvestmentSectorAxis = us-gaap_VentureCapitalFundsMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember	6,606us-gaap_OtherInvestments / invest_InvestmentSectorAxis = us-gaap_VentureCapitalFundsMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Remaining Commitment	350sigi_RemainingCommitment / invest_InvestmentSectorAxis = us-gaap_VentureCapitalFundsMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Other Securities [Member]
Carrying Value	10,746us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_OtherSecuritiesMember	6,235us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_OtherSecuritiesMember
Remaining Commitment	$ 2,001sigi_RemainingCommitment / invest_InvestmentSectorAxis = sigi_OtherSecuritiesMember

X

- Definition

This item represents contractual obligations to invest additional amounts on our other investment portfolio. These additional investments may or may not be required.

+ References

No definition available.

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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

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