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Segment Information (Income Before Federal Income Tax) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Segment Reporting Information [Line Items]
Net investment income earned	$ 26,917us-gaap_NetInvestmentIncome	$ 35,534us-gaap_NetInvestmentIncome
Net realized investment gains	18,883us-gaap_RealizedInvestmentGainsLosses	7,218us-gaap_RealizedInvestmentGainsLosses
Total investment revenues	45,800sigi_TotalInvestmentRevenues	42,752sigi_TotalInvestmentRevenues
Tax on investment income	12,318sigi_Taxoninvestmentincome	11,575sigi_Taxoninvestmentincome
Total investment income after federal income tax	33,482sigi_TotalInvestmentIncomeAfterFederalIncomeTax	31,177sigi_TotalInvestmentIncomeAfterFederalIncomeTax
Standard Commercial Lines [Member]
Segment Reporting Information [Line Items]
Underwriting gain (loss), before federal income tax	30,036sigi_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	$ 975sigi_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

X

- 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.

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- Definition

Taxes incurred on investment income

+ References

No definition available.

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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.

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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.

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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.

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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.

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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

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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

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