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Investments (Aggregated Income Statement Summarized Financial Information for Partnerhips in our Alternative Investment Portfolio) (Details) (USD $) | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014 | Dec. 31, 2014 | Dec. 31, 2013
Schedule of Equity Method Investments [Line Items] |  |  |  |
Net investment income earned | $ 26,917,000 us-gaap_NetInvestmentIncome | $ 35,534,000 us-gaap_NetInvestmentIncome |  |
Alternative Investments [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Net investment income earned | (3,500,000)us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember | 5,200,000 us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember |  |
Investments accounted for under the equity method [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Net Investment Income |  |  | 77,000,000 sigi_NetInvestmentIncomeofEquityMethodInvestees / us-gaap_InvestmentTypeAxis = sigi_InvestmentsAccountedForUnderTheEquityMethodMember | 65,100,000 sigi_NetInvestmentIncomeofEquityMethodInvestees / us-gaap_InvestmentTypeAxis = sigi_InvestmentsAccountedForUnderTheEquityMethodMember
Realized gains |  |  | 160,500,000 sigi_RealizedGainsLossesOfEquityMethodInvestees / us-gaap_InvestmentTypeAxis = sigi_InvestmentsAccountedForUnderTheEquityMethodMember | 63,300,000 sigi_RealizedGainsLossesOfEquityMethodInvestees / us-gaap_InvestmentTypeAxis = sigi_InvestmentsAccountedForUnderTheEquityMethodMember
Net change in unrealized (depreciation) appreciation |  |  | (518,000,000)sigi_NetChangeInUnrealizedAppreciationDepreciationOfEquityMethodInvestees / us-gaap_InvestmentTypeAxis = sigi_InvestmentsAccountedForUnderTheEquityMethodMember | 505,900,000 sigi_NetChangeInUnrealizedAppreciationDepreciationOfEquityMethodInvestees / us-gaap_InvestmentTypeAxis = sigi_InvestmentsAccountedForUnderTheEquityMethodMember
Net income (loss) |  |  | $ (280,500,000)us-gaap_EquityMethodInvestmentSummarizedFinancialInformationNetIncomeLoss / us-gaap_InvestmentTypeAxis = sigi_InvestmentsAccountedForUnderTheEquityMethodMember | $ 634,300,000 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationNetIncomeLoss / us-gaap_InvestmentTypeAxis = sigi_InvestmentsAccountedForUnderTheEquityMethodMember

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