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Segment Information (Revenue by Segment) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Segment Reporting Information [Line Items]
Total net premiums earned	$ 476,123us-gaap_PremiumsEarnedNetPropertyAndCasualty	$ 456,495us-gaap_PremiumsEarnedNetPropertyAndCasualty
Investment Income, Net	26,917us-gaap_InvestmentIncomeNet	35,534us-gaap_InvestmentIncomeNet
Other income	1,969us-gaap_OtherIncome	9,824us-gaap_OtherIncome
Net realized investment gains	18,883us-gaap_RealizedInvestmentGainsLosses	7,218us-gaap_RealizedInvestmentGainsLosses
Total investment revenues	45,800sigi_TotalInvestmentRevenues	42,752sigi_TotalInvestmentRevenues
Total revenues	523,892us-gaap_Revenues	509,071us-gaap_Revenues
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	523,892us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = us-gaap_OperatingSegmentsMember	509,066us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = us-gaap_OperatingSegmentsMember
Other Income [Member]
Segment Reporting Information [Line Items]
Other income	0us-gaap_OtherIncome / us-gaap_StatementBusinessSegmentsAxis = sigi_OtherRevenueMember	5us-gaap_OtherIncome / us-gaap_StatementBusinessSegmentsAxis = sigi_OtherRevenueMember
Standard Commercial Lines [Member] | Commercial Automobile [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	86,355us-gaap_PremiumsEarnedNetPropertyAndCasualty / sigi_LineOfBusinessBySegmentAxis = sigi_StandardCommercialLinesMember / us-gaap_StatementBusinessSegmentsAxis = sigi_CommercialAutomobileMember	82,216us-gaap_PremiumsEarnedNetPropertyAndCasualty / sigi_LineOfBusinessBySegmentAxis = sigi_StandardCommercialLinesMember / us-gaap_StatementBusinessSegmentsAxis = sigi_CommercialAutomobileMember
Standard Commercial Lines [Member] | Workers Compensation [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	68,477us-gaap_PremiumsEarnedNetPropertyAndCasualty / sigi_LineOfBusinessBySegmentAxis = sigi_StandardCommercialLinesMember / us-gaap_StatementBusinessSegmentsAxis = sigi_WorkersCompensationMember	69,413us-gaap_PremiumsEarnedNetPropertyAndCasualty / sigi_LineOfBusinessBySegmentAxis = sigi_StandardCommercialLinesMember / us-gaap_StatementBusinessSegmentsAxis = sigi_WorkersCompensationMember
Standard Commercial Lines [Member] | General Liability [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	114,971us-gaap_PremiumsEarnedNetPropertyAndCasualty / sigi_LineOfBusinessBySegmentAxis = sigi_StandardCommercialLinesMember / us-gaap_StatementBusinessSegmentsAxis = us-gaap_GeneralLiabilityMember	108,818us-gaap_PremiumsEarnedNetPropertyAndCasualty / sigi_LineOfBusinessBySegmentAxis = sigi_StandardCommercialLinesMember / us-gaap_StatementBusinessSegmentsAxis = us-gaap_GeneralLiabilityMember
Standard Commercial Lines [Member] | Commercial Property [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	64,563us-gaap_PremiumsEarnedNetPropertyAndCasualty / sigi_LineOfBusinessBySegmentAxis = sigi_StandardCommercialLinesMember / us-gaap_StatementBusinessSegmentsAxis = us-gaap_PropertyAndCasualtyCommercialInsuranceProductLineMember	60,186us-gaap_PremiumsEarnedNetPropertyAndCasualty / sigi_LineOfBusinessBySegmentAxis = sigi_StandardCommercialLinesMember / us-gaap_StatementBusinessSegmentsAxis = us-gaap_PropertyAndCasualtyCommercialInsuranceProductLineMember
Standard Commercial Lines [Member] | Business Owners' Policies [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	22,699us-gaap_PremiumsEarnedNetPropertyAndCasualty / sigi_LineOfBusinessBySegmentAxis = sigi_StandardCommercialLinesMember / us-gaap_StatementBusinessSegmentsAxis = sigi_BusinessOwnersPoliciesMember	20,869us-gaap_PremiumsEarnedNetPropertyAndCasualty / sigi_LineOfBusinessBySegmentAxis = sigi_StandardCommercialLinesMember / us-gaap_StatementBusinessSegmentsAxis = sigi_BusinessOwnersPoliciesMember
Standard Commercial Lines [Member] | Bonds Segment [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	5,000us-gaap_PremiumsEarnedNetPropertyAndCasualty / sigi_LineOfBusinessBySegmentAxis = sigi_StandardCommercialLinesMember / us-gaap_StatementBusinessSegmentsAxis = us-gaap_SuretySegmentMember	4,756us-gaap_PremiumsEarnedNetPropertyAndCasualty / sigi_LineOfBusinessBySegmentAxis = sigi_StandardCommercialLinesMember / us-gaap_StatementBusinessSegmentsAxis = us-gaap_SuretySegmentMember
Standard Commercial Lines [Member] | Other Commercial [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	3,468us-gaap_PremiumsEarnedNetPropertyAndCasualty / sigi_LineOfBusinessBySegmentAxis = sigi_StandardCommercialLinesMember / us-gaap_StatementBusinessSegmentsAxis = sigi_OtherCommercialMember	3,183us-gaap_PremiumsEarnedNetPropertyAndCasualty / sigi_LineOfBusinessBySegmentAxis = sigi_StandardCommercialLinesMember / us-gaap_StatementBusinessSegmentsAxis = sigi_OtherCommercialMember
Standard Commercial Lines [Member] | Miscellaneous Income [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	1,669us-gaap_PremiumsEarnedNetPropertyAndCasualty / sigi_LineOfBusinessBySegmentAxis = sigi_StandardCommercialLinesMember / us-gaap_StatementBusinessSegmentsAxis = sigi_MiscellaneousIncomeMember	8,914us-gaap_PremiumsEarnedNetPropertyAndCasualty / sigi_LineOfBusinessBySegmentAxis = sigi_StandardCommercialLinesMember / us-gaap_StatementBusinessSegmentsAxis = sigi_MiscellaneousIncomeMember
Standard Commercial Lines [Member] | Total Standard Commercial Lines [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	367,202us-gaap_PremiumsEarnedNetPropertyAndCasualty / sigi_LineOfBusinessBySegmentAxis = sigi_StandardCommercialLinesMember / us-gaap_StatementBusinessSegmentsAxis = sigi_TotalCommercialLinesMember	358,355us-gaap_PremiumsEarnedNetPropertyAndCasualty / sigi_LineOfBusinessBySegmentAxis = sigi_StandardCommercialLinesMember / us-gaap_StatementBusinessSegmentsAxis = sigi_TotalCommercialLinesMember
Standard Personal Lines [Member] | Miscellaneous Income [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	300us-gaap_PremiumsEarnedNetPropertyAndCasualty / sigi_LineOfBusinessBySegmentAxis = sigi_StandardPersonalLinesMember / us-gaap_StatementBusinessSegmentsAxis = sigi_MiscellaneousIncomeMember	905us-gaap_PremiumsEarnedNetPropertyAndCasualty / sigi_LineOfBusinessBySegmentAxis = sigi_StandardPersonalLinesMember / us-gaap_StatementBusinessSegmentsAxis = sigi_MiscellaneousIncomeMember
Standard Personal Lines [Member] | Personal Automobile [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	37,010us-gaap_PremiumsEarnedNetPropertyAndCasualty / sigi_LineOfBusinessBySegmentAxis = sigi_StandardPersonalLinesMember / us-gaap_StatementBusinessSegmentsAxis = sigi_PersonalAutomobileMember	38,226us-gaap_PremiumsEarnedNetPropertyAndCasualty / sigi_LineOfBusinessBySegmentAxis = sigi_StandardPersonalLinesMember / us-gaap_StatementBusinessSegmentsAxis = sigi_PersonalAutomobileMember
Standard Personal Lines [Member] | Homeowners [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	33,721us-gaap_PremiumsEarnedNetPropertyAndCasualty / sigi_LineOfBusinessBySegmentAxis = sigi_StandardPersonalLinesMember / us-gaap_StatementBusinessSegmentsAxis = sigi_HomeownersMember	33,298us-gaap_PremiumsEarnedNetPropertyAndCasualty / sigi_LineOfBusinessBySegmentAxis = sigi_StandardPersonalLinesMember / us-gaap_StatementBusinessSegmentsAxis = sigi_HomeownersMember
Standard Personal Lines [Member] | Other Personal [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	1,748us-gaap_PremiumsEarnedNetPropertyAndCasualty / sigi_LineOfBusinessBySegmentAxis = sigi_StandardPersonalLinesMember / us-gaap_StatementBusinessSegmentsAxis = sigi_OtherPersonalMember	3,294us-gaap_PremiumsEarnedNetPropertyAndCasualty / sigi_LineOfBusinessBySegmentAxis = sigi_StandardPersonalLinesMember / us-gaap_StatementBusinessSegmentsAxis = sigi_OtherPersonalMember
Standard Personal Lines [Member] | Total Standard Personal Lines [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	72,779us-gaap_PremiumsEarnedNetPropertyAndCasualty / sigi_LineOfBusinessBySegmentAxis = sigi_StandardPersonalLinesMember / us-gaap_StatementBusinessSegmentsAxis = us-gaap_PropertyAndCasualtyPersonalInsuranceProductLineMember	75,723us-gaap_PremiumsEarnedNetPropertyAndCasualty / sigi_LineOfBusinessBySegmentAxis = sigi_StandardPersonalLinesMember / us-gaap_StatementBusinessSegmentsAxis = us-gaap_PropertyAndCasualtyPersonalInsuranceProductLineMember
Excess And Surplus Operations [Member] | Commercial Automobile [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	1,730us-gaap_PremiumsEarnedNetPropertyAndCasualty / sigi_LineOfBusinessBySegmentAxis = sigi_ExcessAndSurplusOperationsMember / us-gaap_StatementBusinessSegmentsAxis = sigi_CommercialAutomobileMember	1,188us-gaap_PremiumsEarnedNetPropertyAndCasualty / sigi_LineOfBusinessBySegmentAxis = sigi_ExcessAndSurplusOperationsMember / us-gaap_StatementBusinessSegmentsAxis = sigi_CommercialAutomobileMember
Excess And Surplus Operations [Member] | General Liability [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	26,726us-gaap_PremiumsEarnedNetPropertyAndCasualty / sigi_LineOfBusinessBySegmentAxis = sigi_ExcessAndSurplusOperationsMember / us-gaap_StatementBusinessSegmentsAxis = us-gaap_GeneralLiabilityMember	21,931us-gaap_PremiumsEarnedNetPropertyAndCasualty / sigi_LineOfBusinessBySegmentAxis = sigi_ExcessAndSurplusOperationsMember / us-gaap_StatementBusinessSegmentsAxis = us-gaap_GeneralLiabilityMember
Excess And Surplus Operations [Member] | Commercial Property [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	9,655us-gaap_PremiumsEarnedNetPropertyAndCasualty / sigi_LineOfBusinessBySegmentAxis = sigi_ExcessAndSurplusOperationsMember / us-gaap_StatementBusinessSegmentsAxis = us-gaap_PropertyAndCasualtyCommercialInsuranceProductLineMember	9,117us-gaap_PremiumsEarnedNetPropertyAndCasualty / sigi_LineOfBusinessBySegmentAxis = sigi_ExcessAndSurplusOperationsMember / us-gaap_StatementBusinessSegmentsAxis = us-gaap_PropertyAndCasualtyCommercialInsuranceProductLineMember
Excess And Surplus Operations [Member] | Total Standard Personal Lines [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	$ 38,111us-gaap_PremiumsEarnedNetPropertyAndCasualty / sigi_LineOfBusinessBySegmentAxis = sigi_ExcessAndSurplusOperationsMember / us-gaap_StatementBusinessSegmentsAxis = us-gaap_PropertyAndCasualtyPersonalInsuranceProductLineMember	$ 32,236us-gaap_PremiumsEarnedNetPropertyAndCasualty / sigi_LineOfBusinessBySegmentAxis = sigi_ExcessAndSurplusOperationsMember / us-gaap_StatementBusinessSegmentsAxis = us-gaap_PropertyAndCasualtyPersonalInsuranceProductLineMember

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