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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 | $ 38,637 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 |  |  |
Total investment revenues |  |  |  |  |  |  |  |  | 165,307 sigi_TotalInvestmentRevenues | 155,375 sigi_TotalInvestmentRevenues | 140,865 sigi_TotalInvestmentRevenues
Total all segments |  |  |  |  |  |  |  |  | 243,450 sigi_TotalSegmentIncomeLossFromContinuingOperationsBeforeFederalIncomeTax | 194,141 sigi_TotalSegmentIncomeLossFromContinuingOperationsBeforeFederalIncomeTax | 76,858 sigi_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 |  |  |  |  |  |  |  |  | 197,131 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest | 143,802 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest | 37,635 us-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 |  |  |  |  |  |  |  |  | 61,221 sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes / us-gaap_StatementBusinessSegmentsAxis = sigi_StandardCommercialLinesMember | 33,856 sigi_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 |  |  |  |  |  |  |  |  | $ 16,536 sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes / us-gaap_StatementBusinessSegmentsAxis = sigi_StandardPersonalLinesMember | $ 8,645 sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes / us-gaap_StatementBusinessSegmentsAxis = sigi_StandardPersonalLinesMember | $ (3,514)sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes / us-gaap_StatementBusinessSegmentsAxis = sigi_StandardPersonalLinesMember

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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. + Details
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