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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	$ 38,637sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes	$ 34,437sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes	$ 10,084sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes	$ (5,015)sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes	$ 11,971sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes	$ 10,151sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes	$ 4,483sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes	$ 12,161sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes
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 investment gains									26,599us-gaap_RealizedInvestmentGainsLosses	20,732us-gaap_RealizedInvestmentGainsLosses	8,988us-gaap_RealizedInvestmentGainsLosses
Total investment revenues									165,307sigi_TotalInvestmentRevenues	155,375sigi_TotalInvestmentRevenues	140,865sigi_TotalInvestmentRevenues
Tax on investment income									43,811sigi_Taxoninvestmentincome	40,489sigi_Taxoninvestmentincome	34,758sigi_Taxoninvestmentincome
Total investment income, after federal income tax									121,496sigi_TotalInvestmentIncomeAfterFederalIncomeTax	114,886sigi_TotalInvestmentIncomeAfterFederalIncomeTax	106,107sigi_TotalInvestmentIncomeAfterFederalIncomeTax
Standard Commercial Lines [Member]
Underwriting gain (loss), before federal income tax									61,221sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes / sigi_LineOfBusinessBySegmentAxis = sigi_StandardCommercialLinesMember	33,856sigi_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									16,536sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes / sigi_LineOfBusinessBySegmentAxis = sigi_StandardPersonalLinesMember	8,645sigi_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									$ 386sigi_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.

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Taxes paid on investment income

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The income earned from investments in securities and other investments including dividends, interest, and returns on other investments, including realized gains/losses, after-tax

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The income earned from investments in securities and other investments including dividends, interest, and returns on other investments, including realized gains/losses.

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Net premiums earned and other insurance income less net loss and loss expenses incurred, loss adjustment expenses, policy acquisition costs and other underwriting expenses.

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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.

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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.

+ 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

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-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913



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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.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

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-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913



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