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Segment Information (Income Before Federal Income Tax) (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Segment Reporting Information [Line Items] |  |
Net investment income earned | $ 26,917 us-gaap_NetInvestmentIncome | $ 35,534 us-gaap_NetInvestmentIncome
Net realized investment gains | 18,883 us-gaap_RealizedInvestmentGainsLosses | 7,218 us-gaap_RealizedInvestmentGainsLosses
Total investment revenues | 45,800 sigi_TotalInvestmentRevenues | 42,752 sigi_TotalInvestmentRevenues
Tax on investment income | 12,318 sigi_Taxoninvestmentincome | 11,575 sigi_Taxoninvestmentincome
Total investment income after federal income tax | 33,482 sigi_TotalInvestmentIncomeAfterFederalIncomeTax | 31,177 sigi_TotalInvestmentIncomeAfterFederalIncomeTax
Standard Commercial Lines [Member] |  |
Segment Reporting Information [Line Items] |  |
Underwriting gain (loss), before federal income tax | 30,036 sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes / us-gaap_StatementBusinessSegmentsAxis = sigi_StandardCommercialLinesMember | (3,630)sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes / us-gaap_StatementBusinessSegmentsAxis = sigi_StandardCommercialLinesMember
GAAP combined ratio | 91.80%us-gaap_CombinedRatio / us-gaap_StatementBusinessSegmentsAxis = sigi_StandardCommercialLinesMember | 101.00%us-gaap_CombinedRatio / us-gaap_StatementBusinessSegmentsAxis = sigi_StandardCommercialLinesMember
Statutory combined ratio | 89.70%sigi_StatutoryCombinedRatio / us-gaap_StatementBusinessSegmentsAxis = sigi_StandardCommercialLinesMember | 100.30%sigi_StatutoryCombinedRatio / us-gaap_StatementBusinessSegmentsAxis = sigi_StandardCommercialLinesMember
Standard Personal Lines [Member] |  |
Segment Reporting Information [Line Items] |  |
Underwriting gain (loss), before federal income tax | (2,466)sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes / us-gaap_StatementBusinessSegmentsAxis = sigi_StandardPersonalLinesMember | (2,360)sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes / us-gaap_StatementBusinessSegmentsAxis = sigi_StandardPersonalLinesMember
GAAP combined ratio | 103.40%us-gaap_CombinedRatio / us-gaap_StatementBusinessSegmentsAxis = sigi_StandardPersonalLinesMember | 103.20%us-gaap_CombinedRatio / us-gaap_StatementBusinessSegmentsAxis = sigi_StandardPersonalLinesMember
Statutory combined ratio | 105.10%sigi_StatutoryCombinedRatio / us-gaap_StatementBusinessSegmentsAxis = sigi_StandardPersonalLinesMember | 104.50%sigi_StatutoryCombinedRatio / us-gaap_StatementBusinessSegmentsAxis = sigi_StandardPersonalLinesMember
Excess And Surplus Operations [Member] |  |
Segment Reporting Information [Line Items] |  |
Underwriting gain (loss), before federal income tax | $ (1,549)sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes / us-gaap_StatementBusinessSegmentsAxis = sigi_ExcessAndSurplusOperationsMember | $ 975 sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes / us-gaap_StatementBusinessSegmentsAxis = sigi_ExcessAndSurplusOperationsMember
GAAP combined ratio | 104.10%us-gaap_CombinedRatio / us-gaap_StatementBusinessSegmentsAxis = sigi_ExcessAndSurplusOperationsMember | 97.00%us-gaap_CombinedRatio / us-gaap_StatementBusinessSegmentsAxis = sigi_ExcessAndSurplusOperationsMember
Statutory combined ratio | 102.10%sigi_StatutoryCombinedRatio / us-gaap_StatementBusinessSegmentsAxis = sigi_ExcessAndSurplusOperationsMember | 97.90%sigi_StatutoryCombinedRatio / us-gaap_StatementBusinessSegmentsAxis = sigi_ExcessAndSurplusOperationsMember

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- Definition The statutory combined ratio (expressed as a percentage) is the sum of: (i) the loss and loss expense ratio; (ii) the underwriting ratio; and (iii) the dividend ratio. The loss and loss expense ratio as well as the dividend ratio is calculated by dividing loss and loss expenses, as well as dividends to policyholders, by net premiums earned. The underwriting expense ratio is calculated by dividing underwriting expenses by net premiums written. + References No definition available. + Details
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- Definition Taxes incurred on investment income + References No definition available. + Details
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- Definition The income earned from investments in securities and other investments including dividends, interest, and returns on other investments, including realized gains/losses, after-tax + References No definition available. + Details
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- Definition The income earned from investments in securities and other investments including dividends, interest, and returns on other investments, including realized gains/losses. + References No definition available. + Details
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- Definition Net premiums earned and other insurance income less net loss and loss expenses incurred, loss adjustment expenses, policy acquisition costs and other underwriting expenses. + References No definition available. + Details
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- Definition Ratio of non-life losses and loss expenses, non-life acquisition expenses and non-life general and administrative expenses, to non-life net premiums earned. + References No definition available. + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 16 -Article 12 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.2) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 + Details
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- Definition The net realized gains or losses on investments during the period, not including gains or losses on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.3(a)) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 325 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6485959&loc=d3e22529-158502 + Details
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