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Segment Information (Reconciliation of Segment Results to 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
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
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
Total investment revenues									165,307sigi_TotalInvestmentRevenues	155,375sigi_TotalInvestmentRevenues	140,865sigi_TotalInvestmentRevenues
Total all segments									243,450sigi_TotalSegmentIncomeLossFromContinuingOperationsBeforeFederalIncomeTax	194,141sigi_TotalSegmentIncomeLossFromContinuingOperationsBeforeFederalIncomeTax	76,858sigi_TotalSegmentIncomeLossFromContinuingOperationsBeforeFederalIncomeTax
Interest expense									(22,086)us-gaap_InterestExpenseDebt	(22,538)us-gaap_InterestExpenseDebt	(18,872)us-gaap_InterestExpenseDebt
Other General Expense									(24,233)us-gaap_OtherGeneralExpense	(27,801)us-gaap_OtherGeneralExpense	(20,351)us-gaap_OtherGeneralExpense
Income from continuing operations, before federal income tax									197,131us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	143,802us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	37,635us-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									61,221sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes / us-gaap_StatementBusinessSegmentsAxis = sigi_StandardCommercialLinesMember	33,856sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes / us-gaap_StatementBusinessSegmentsAxis = sigi_StandardCommercialLinesMember	(40,935)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									$ 16,536sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes / us-gaap_StatementBusinessSegmentsAxis = sigi_StandardPersonalLinesMember	$ 8,645sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes / us-gaap_StatementBusinessSegmentsAxis = sigi_StandardPersonalLinesMember	$ (3,514)sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes / us-gaap_StatementBusinessSegmentsAxis = sigi_StandardPersonalLinesMember

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

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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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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.

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

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