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Segment Information (Reconciliation of Segment Results to Income Before Federal Income Tax (Details)) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Investment income, before federal income tax	$ 45,800sigi_TotalInvestmentRevenues	$ 42,752sigi_TotalInvestmentRevenues
Interest expense	(5,479)us-gaap_InterestExpenseDebt	(5,561)us-gaap_InterestExpenseDebt
General corporate and other expenses	(10,052)us-gaap_OtherGeneralExpense	(7,092)us-gaap_OtherGeneralExpense
Income before federal income tax	56,290us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	25,084us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest
Standard Commercial Lines [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [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
Standard Personal Lines [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [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
Excess And Surplus Operations [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [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
Operating Segments [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total all segments	$ 71,821sigi_TotalSegmentIncomeLossFromContinuingOperationsBeforeFederalIncomeTax / us-gaap_StatementBusinessSegmentsAxis = us-gaap_OperatingSegmentsMember	$ 37,737sigi_TotalSegmentIncomeLossFromContinuingOperationsBeforeFederalIncomeTax / us-gaap_StatementBusinessSegmentsAxis = us-gaap_OperatingSegmentsMember

X

- 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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X

- Definition

Total of all reportable segments' income or loss from continuing operations, before federal income taxes.

+ References

No definition available.

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X

- 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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X

- Definition

This element represents the income or loss from continuing operations attributable to the economic entity which may also be defined as revenue less expenses from ongoing operations, after income or loss from equity method investments, but before income taxes, extraordinary items, and noncontrolling interest.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 940

-SubTopic 20

-Section 25

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=51673353&loc=d3e41242-110953



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 10

-Article 5

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- Definition

Amount of the cost of borrowed funds accounted for as interest expense for debt.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.8)

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

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- Definition

Amount of general expenses not normally included in Other Operating Costs and Expenses.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.6)

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

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