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v2.4.1.9

Segment Information (Income From Continuing Operations, Before Federal Income Tax) (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
Underwriting gain (loss), before federal income tax | $ 38,637 sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes | $ 34,437 sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes | $ 10,084 sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes | $ (5,015)sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes | $ 11,971 sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes | $ 10,151 sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes | $ 4,483 sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes | $ 12,161 sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes |  |  |
Net Investment Income | 32,108 us-gaap_NetInvestmentIncome | 34,292 us-gaap_NetInvestmentIncome | 36,774 us-gaap_NetInvestmentIncome | 35,534 us-gaap_NetInvestmentIncome | 35,313 us-gaap_NetInvestmentIncome | 32,457 us-gaap_NetInvestmentIncome | 34,003 us-gaap_NetInvestmentIncome | 32,870 us-gaap_NetInvestmentIncome | 138,708 us-gaap_NetInvestmentIncome | 134,643 us-gaap_NetInvestmentIncome | 131,877 us-gaap_NetInvestmentIncome
Net realized investment gains |  |  |  |  |  |  |  |  | 26,599 us-gaap_RealizedInvestmentGainsLosses | 20,732 us-gaap_RealizedInvestmentGainsLosses | 8,988 us-gaap_RealizedInvestmentGainsLosses
Total investment revenues |  |  |  |  |  |  |  |  | 165,307 sigi_TotalInvestmentRevenues | 155,375 sigi_TotalInvestmentRevenues | 140,865 sigi_TotalInvestmentRevenues
Tax on investment income |  |  |  |  |  |  |  |  | 43,811 sigi_Taxoninvestmentincome | 40,489 sigi_Taxoninvestmentincome | 34,758 sigi_Taxoninvestmentincome
Total investment income, after federal income tax |  |  |  |  |  |  |  |  | 121,496 sigi_TotalInvestmentIncomeAfterFederalIncomeTax | 114,886 sigi_TotalInvestmentIncomeAfterFederalIncomeTax | 106,107 sigi_TotalInvestmentIncomeAfterFederalIncomeTax
Standard Commercial Lines [Member] |  |  |  |  |  |  |  |  |  |  |
Underwriting gain (loss), before federal income tax |  |  |  |  |  |  |  |  | 61,221 sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes / sigi_LineOfBusinessBySegmentAxis = sigi_StandardCommercialLinesMember | 33,856 sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes / sigi_LineOfBusinessBySegmentAxis = sigi_StandardCommercialLinesMember | (40,935)sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes / sigi_LineOfBusinessBySegmentAxis = sigi_StandardCommercialLinesMember
GAAP Combined Ratio |  |  |  |  |  |  |  |  | 95.70%us-gaap_CombinedRatio / sigi_LineOfBusinessBySegmentAxis = sigi_StandardCommercialLinesMember | 97.40%us-gaap_CombinedRatio / sigi_LineOfBusinessBySegmentAxis = sigi_StandardCommercialLinesMember | 103.30%us-gaap_CombinedRatio / sigi_LineOfBusinessBySegmentAxis = sigi_StandardCommercialLinesMember
Statutory Combined Ratio |  |  |  |  |  |  |  |  | 95.50%sigi_StatutoryCombinedRatio / sigi_LineOfBusinessBySegmentAxis = sigi_StandardCommercialLinesMember | 97.10%sigi_StatutoryCombinedRatio / sigi_LineOfBusinessBySegmentAxis = sigi_StandardCommercialLinesMember | 103.00%sigi_StatutoryCombinedRatio / sigi_LineOfBusinessBySegmentAxis = sigi_StandardCommercialLinesMember
Standard Personal Lines [Member] |  |  |  |  |  |  |  |  |  |  |
Underwriting gain (loss), before federal income tax |  |  |  |  |  |  |  |  | 16,536 sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes / sigi_LineOfBusinessBySegmentAxis = sigi_StandardPersonalLinesMember | 8,645 sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes / sigi_LineOfBusinessBySegmentAxis = sigi_StandardPersonalLinesMember | (3,514)sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes / sigi_LineOfBusinessBySegmentAxis = sigi_StandardPersonalLinesMember
GAAP Combined Ratio |  |  |  |  |  |  |  |  | 94.40%us-gaap_CombinedRatio / sigi_LineOfBusinessBySegmentAxis = sigi_StandardPersonalLinesMember | 97.10%us-gaap_CombinedRatio / sigi_LineOfBusinessBySegmentAxis = sigi_StandardPersonalLinesMember | 101.30%us-gaap_CombinedRatio / sigi_LineOfBusinessBySegmentAxis = sigi_StandardPersonalLinesMember
Statutory Combined Ratio |  |  |  |  |  |  |  |  | 94.50%sigi_StatutoryCombinedRatio / sigi_LineOfBusinessBySegmentAxis = sigi_StandardPersonalLinesMember | 96.90%sigi_StatutoryCombinedRatio / sigi_LineOfBusinessBySegmentAxis = sigi_StandardPersonalLinesMember | 100.70%sigi_StatutoryCombinedRatio / sigi_LineOfBusinessBySegmentAxis = sigi_StandardPersonalLinesMember
Excess And Surplus Operations [Member] |  |  |  |  |  |  |  |  |  |  |
Underwriting gain (loss), before federal income tax |  |  |  |  |  |  |  |  | $ 386 sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes / sigi_LineOfBusinessBySegmentAxis = sigi_ExcessAndSurplusOperationsMember | $ (3,735)sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes / sigi_LineOfBusinessBySegmentAxis = sigi_ExcessAndSurplusOperationsMember | $ (19,558)sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes / sigi_LineOfBusinessBySegmentAxis = sigi_ExcessAndSurplusOperationsMember
GAAP Combined Ratio |  |  |  |  |  |  |  |  | 99.70%us-gaap_CombinedRatio / sigi_LineOfBusinessBySegmentAxis = sigi_ExcessAndSurplusOperationsMember | 103.00%us-gaap_CombinedRatio / sigi_LineOfBusinessBySegmentAxis = sigi_ExcessAndSurplusOperationsMember | 124.70%us-gaap_CombinedRatio / sigi_LineOfBusinessBySegmentAxis = sigi_ExcessAndSurplusOperationsMember
Statutory Combined Ratio |  |  |  |  |  |  |  |  | 99.20%sigi_StatutoryCombinedRatio / sigi_LineOfBusinessBySegmentAxis = sigi_ExcessAndSurplusOperationsMember | 102.90%sigi_StatutoryCombinedRatio / sigi_LineOfBusinessBySegmentAxis = sigi_ExcessAndSurplusOperationsMember | 118.80%sigi_StatutoryCombinedRatio / sigi_LineOfBusinessBySegmentAxis = 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 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 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 2 -Article 7 + 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 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 3 -Subparagraph a -Article 7 + Details
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