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Segment Information (Revenue From Continuing Operations by Segment) (Details) (USD $)
In Thousands, unless otherwise specified

3 Months Ended

12 Months Ended

Dec. 31, 2014

Sep. 30, 2014

Jun. 30, 2014

Mar. 31, 2014

Dec. 31, 2013

Sep. 30, 2013

Jun. 30, 2013

Mar. 31, 2013

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Revenue from External Customer [Line Items]

Net premiums earned

$ 469,850us-gaap_PremiumsEarnedNetPropertyAndCasualty

$ 462,639us-gaap_PremiumsEarnedNetPropertyAndCasualty

$ 463,625us-gaap_PremiumsEarnedNetPropertyAndCasualty

$ 456,495us-gaap_PremiumsEarnedNetPropertyAndCasualty

$ 451,312us-gaap_PremiumsEarnedNetPropertyAndCasualty

$ 437,568us-gaap_PremiumsEarnedNetPropertyAndCasualty

$ 426,252us-gaap_PremiumsEarnedNetPropertyAndCasualty

$ 420,940us-gaap_PremiumsEarnedNetPropertyAndCasualty

$ 1,852,609us-gaap_PremiumsEarnedNetPropertyAndCasualty

$ 1,736,072us-gaap_PremiumsEarnedNetPropertyAndCasualty

$ 1,584,119us-gaap_PremiumsEarnedNetPropertyAndCasualty

Other income

16,945us-gaap_OtherIncome

12,294us-gaap_OtherIncome

9,118us-gaap_OtherIncome

Net Investment Income

32,108us-gaap_NetInvestmentIncome

34,292us-gaap_NetInvestmentIncome

36,774us-gaap_NetInvestmentIncome

35,534us-gaap_NetInvestmentIncome

35,313us-gaap_NetInvestmentIncome

32,457us-gaap_NetInvestmentIncome

34,003us-gaap_NetInvestmentIncome

32,870us-gaap_NetInvestmentIncome

138,708us-gaap_NetInvestmentIncome

134,643us-gaap_NetInvestmentIncome

131,877us-gaap_NetInvestmentIncome

Net realized (gains) losses

26,599us-gaap_RealizedInvestmentGainsLosses

20,732us-gaap_RealizedInvestmentGainsLosses

8,988us-gaap_RealizedInvestmentGainsLosses

Total investment revenues

165,307sigi_TotalInvestmentRevenues

155,375sigi_TotalInvestmentRevenues

140,865sigi_TotalInvestmentRevenues

Total all segments

2,034,514sigi_SegmentReportingRevenue

1,903,648sigi_SegmentReportingRevenue

1,733,811sigi_SegmentReportingRevenue

Total revenues

2,034,861us-gaap_Revenues

1,903,741us-gaap_Revenues

1,734,102us-gaap_Revenues

Miscellaneous Income [Member]

Revenue from External Customer [Line Items]

Other income

16,598us-gaap_OtherIncome
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_MiscellaneousIncomeMember

^[1]

12,201us-gaap_OtherIncome
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_MiscellaneousIncomeMember

^[2]

8,827us-gaap_OtherIncome
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_MiscellaneousIncomeMember

^[3]

Other Income [Member]

Revenue from External Customer [Line Items]

Other income

347us-gaap_OtherIncome
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_OtherRevenueMember

93us-gaap_OtherIncome
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_OtherRevenueMember

291us-gaap_OtherIncome
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_OtherRevenueMember

Standard Commercial Lines [Member] | Commercial Automobile [Member]

Revenue from External Customer [Line Items]

Net premiums earned

333,310us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardCommercialLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_CommercialAutomobileMember

310,994us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardCommercialLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_CommercialAutomobileMember

288,010us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardCommercialLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_CommercialAutomobileMember

Standard Commercial Lines [Member] | Workers' Compensation [Member]

Revenue from External Customer [Line Items]

Net premiums earned

274,585us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardCommercialLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_WorkersCompensationMember

267,612us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardCommercialLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_WorkersCompensationMember

262,108us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardCommercialLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_WorkersCompensationMember

Standard Commercial Lines [Member] | General Liability [Member]

Revenue from External Customer [Line Items]

Net premiums earned

444,938us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardCommercialLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_GeneralLiabilityMember

405,322us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardCommercialLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_GeneralLiabilityMember

373,381us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardCommercialLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_GeneralLiabilityMember

Standard Commercial Lines [Member] | Commercial Property [Member]

Revenue from External Customer [Line Items]

Net premiums earned

244,792us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardCommercialLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_PropertyAndCasualtyCommercialInsuranceProductLineMember

224,412us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardCommercialLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_PropertyAndCasualtyCommercialInsuranceProductLineMember

202,340us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardCommercialLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_PropertyAndCasualtyCommercialInsuranceProductLineMember

Standard Commercial Lines [Member] | Business Owners' Policies [Member]

Revenue from External Customer [Line Items]

Net premiums earned

85,788us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardCommercialLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_BusinessOwnersPoliciesMember

77,097us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardCommercialLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_BusinessOwnersPoliciesMember

68,462us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardCommercialLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_BusinessOwnersPoliciesMember

Standard Commercial Lines [Member] | Bonds Segment [Member]

Revenue from External Customer [Line Items]

Net premiums earned

19,288us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardCommercialLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_SuretySegmentMember

19,000us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardCommercialLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_SuretySegmentMember

18,891us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardCommercialLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_SuretySegmentMember

Standard Commercial Lines [Member] | Other Commercial [Member]

Revenue from External Customer [Line Items]

Net premiums earned

13,011us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardCommercialLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_OtherCommercialMember

12,182us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardCommercialLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_OtherCommercialMember

12,143us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardCommercialLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_OtherCommercialMember

Standard Commercial Lines [Member] | Total standard Commercial Lines [Member]

Revenue from External Customer [Line Items]

Net premiums earned

1,430,459us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardCommercialLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_TotalCommercialLinesMember

1,326,872us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardCommercialLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_TotalCommercialLinesMember

1,232,338us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardCommercialLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_TotalCommercialLinesMember

Standard Commercial Lines [Member] | Miscellaneous Income [Member]

Revenue from External Customer [Line Items]

Other income

14,747us-gaap_OtherIncome
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardCommercialLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_MiscellaneousIncomeMember

10,253us-gaap_OtherIncome
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardCommercialLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_MiscellaneousIncomeMember

7,003us-gaap_OtherIncome
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardCommercialLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_MiscellaneousIncomeMember

Standard Personal Lines [Member] | Personal Automobile [Member]

Revenue from External Customer [Line Items]

Net premiums earned

151,317us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardPersonalLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_PersonalAutomobileMember

152,005us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardPersonalLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_PersonalAutomobileMember

152,142us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardPersonalLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_PersonalAutomobileMember

Standard Personal Lines [Member] | Homeowners [Member]

Revenue from External Customer [Line Items]

Net premiums earned

134,273us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardPersonalLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_HomeownersMember

127,991us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardPersonalLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_HomeownersMember

113,850us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardPersonalLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_HomeownersMember

Standard Personal Lines [Member] | Other Personal [Member]

Revenue from External Customer [Line Items]

Net premiums earned

11,157us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardPersonalLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_OtherPersonalMember

14,336us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardPersonalLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_OtherPersonalMember

13,563us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardPersonalLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_OtherPersonalMember

Standard Personal Lines [Member] | Total Standard Personal Lines [Member]

Revenue from External Customer [Line Items]

Net premiums earned

298,581us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardPersonalLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_PropertyAndCasualtyPersonalInsuranceProductLineMember

296,280us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardPersonalLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_PropertyAndCasualtyPersonalInsuranceProductLineMember

281,379us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardPersonalLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_PropertyAndCasualtyPersonalInsuranceProductLineMember

Standard Personal Lines [Member] | Miscellaneous Income [Member]

Revenue from External Customer [Line Items]

Other income

1,834us-gaap_OtherIncome
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardPersonalLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_MiscellaneousIncomeMember

1,948us-gaap_OtherIncome
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardPersonalLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_MiscellaneousIncomeMember

1,824us-gaap_OtherIncome
/ sigi_LineOfBusinessBySegmentAxis
= sigi_StandardPersonalLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_MiscellaneousIncomeMember

E&S Lines [Member] | Commercial Automobile [Member]

Revenue from External Customer [Line Items]

Net premiums earned

5,436us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_ESLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_CommercialAutomobileMember

4,306us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_ESLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_CommercialAutomobileMember

1,810us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_ESLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_CommercialAutomobileMember

E&S Lines [Member] | General Liability [Member]

Revenue from External Customer [Line Items]

Net premiums earned

96,142us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_ESLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_GeneralLiabilityMember

88,761us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_ESLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_GeneralLiabilityMember

59,721us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_ESLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_GeneralLiabilityMember

E&S Lines [Member] | Commercial Property [Member]

Revenue from External Customer [Line Items]

Net premiums earned

38,572us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_ESLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_PropertyAndCasualtyCommercialInsuranceProductLineMember

32,054us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_ESLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_PropertyAndCasualtyCommercialInsuranceProductLineMember

17,698us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_ESLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_PropertyAndCasualtyCommercialInsuranceProductLineMember

E&S Lines [Member] | Total standard Commercial Lines [Member]

Revenue from External Customer [Line Items]

Net premiums earned

140,167us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_ESLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_TotalCommercialLinesMember

125,121us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_ESLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_TotalCommercialLinesMember

79,229us-gaap_PremiumsEarnedNetPropertyAndCasualty
/ sigi_LineOfBusinessBySegmentAxis
= sigi_ESLinesMember
/ us-gaap_StatementBusinessSegmentsAxis
= sigi_TotalCommercialLinesMember

E&S Lines [Member] | Miscellaneous Income [Member]

Revenue from External Customer [Line Items]

Other income

$ 17us-gaap_OtherIncome
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= sigi_MiscellaneousIncomeMember

$ 0us-gaap_OtherIncome
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$ 0us-gaap_OtherIncome
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[1]	In addition to amounts related to the Standard Commercial Lines, Standard Personal Lines, and E&S Lines, “Other income” and “Other expenses” on the Consolidated Statements of Income includes holding company income and expense amounts of $347 and $24,580, respectively.
[2]	In addition to amounts related to the Standard Commercial Lines, Standard Personal Lines, and E&S Lines, “Other income” and “Other expenses” on the Consolidated Statements of Income includes holding company income and expense amounts of $93 and $27,894, respectively.
[3]	In addition to amounts related to the Standard Commercial Lines, Standard Personal Lines, and E&S Lines, “Other income” and “Other expenses” on the Consolidated Statements of Income includes holding company income and expense amounts of $291 and $20,642, respectively.

X

- Definition

Amount of total revenue for the reportable segments, including revenue from external customers, transactions with other operating segments of the same entity, and (gross) interest revenue. All considered to be "operating revenue" for purposes of the taxonomy.

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Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.

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