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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
Carrying Value	$ 99,203us-gaap_OtherInvestments	$ 107,875us-gaap_OtherInvestments
Alternative Investments [Member]
Carrying Value	92,968us-gaap_OtherInvestments / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember	106,211us-gaap_OtherInvestments / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Remaining Commitment	64,720sigi_RemainingCommitment / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Other Investment Portfolio [Member]
Carrying Value	99,203us-gaap_OtherInvestments / us-gaap_InvestmentTypeAxis = us-gaap_OtherInvestmentsMember	107,875us-gaap_OtherInvestments / us-gaap_InvestmentTypeAxis = us-gaap_OtherInvestmentsMember
Remaining Commitment	68,431sigi_RemainingCommitment / us-gaap_InvestmentTypeAxis = us-gaap_OtherInvestmentsMember
Secondary Private Equity [Member] | Alternative Investments [Member]
Carrying Value	21,807us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_SecondaryPrivateEquityMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember	25,618us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_SecondaryPrivateEquityMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Remaining Commitment	7,001sigi_RemainingCommitment / invest_InvestmentSectorAxis = sigi_SecondaryPrivateEquityMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Private Equity [Member] | Alternative Investments [Member]
Carrying Value	20,126us-gaap_OtherInvestments / invest_InvestmentSectorAxis = us-gaap_PrivateEquityFundsMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember	20,192us-gaap_OtherInvestments / invest_InvestmentSectorAxis = us-gaap_PrivateEquityFundsMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Remaining Commitment	8,890sigi_RemainingCommitment / invest_InvestmentSectorAxis = us-gaap_PrivateEquityFundsMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Energy Power Generation [Member] | Alternative Investments [Member]
Carrying Value	14,445us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_EnergyPowerGenerationMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember	17,361us-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 [Member] | Alternative Investments [Member]
Carrying Value	11,452us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_RealEstateStrategyMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember	11,698us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_RealEstateStrategyMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Remaining Commitment	10,051sigi_RemainingCommitment / invest_InvestmentSectorAxis = sigi_RealEstateStrategyMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Mezzanine Financing [Member] | Alternative Investments [Member]
Carrying Value	9,853us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_MezzanineFinancingMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember	12,738us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_MezzanineFinancingMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Remaining Commitment	13,541sigi_RemainingCommitment / invest_InvestmentSectorAxis = sigi_MezzanineFinancingMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember
Distressed Debt [Member] | Alternative Investments [Member]
Carrying Value	8,679us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_DistressedDebtMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember	11,579us-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 Funds [Member] | Alternative Investments [Member]
Carrying Value	6,606us-gaap_OtherInvestments / invest_InvestmentSectorAxis = us-gaap_VentureCapitalFundsMember / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember	7,025us-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	6,235us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_OtherSecuritiesMember	1,664us-gaap_OtherInvestments / invest_InvestmentSectorAxis = sigi_OtherSecuritiesMember
Remaining Commitment	$ 3,711sigi_RemainingCommitment / invest_InvestmentSectorAxis = sigi_OtherSecuritiesMember

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- 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 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

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