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Investments (Aggregated Income Statement Summarized Financial Information for Partnerhips in our Alternative Investment Portfolio) (Details) (USD $) | 3 Months Ended | 12 Months Ended
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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 | Sep. 30, 2014 | Sep. 30, 2013 | Sep. 30, 2012
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net investment income earned | $ 32,108,000 us-gaap_NetInvestmentIncome | $ 34,292,000 us-gaap_NetInvestmentIncome | $ 36,774,000 us-gaap_NetInvestmentIncome | $ 35,534,000 us-gaap_NetInvestmentIncome | $ 35,313,000 us-gaap_NetInvestmentIncome | $ 32,457,000 us-gaap_NetInvestmentIncome | $ 34,003,000 us-gaap_NetInvestmentIncome | $ 32,870,000 us-gaap_NetInvestmentIncome | $ 138,708,000 us-gaap_NetInvestmentIncome | $ 134,643,000 us-gaap_NetInvestmentIncome | $ 131,877,000 us-gaap_NetInvestmentIncome |  |  |
Investments Accounted For Under The Equity Method [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Investment Income |  |  |  |  |  |  |  |  |  |  |  | 226,000,000 sigi_NetInvestmentIncomeOfEquityMethodInvestees / us-gaap_InvestmentTypeAxis = sigi_InvestmentsAccountedForUnderTheEquityMethodMember | 406,000,000 sigi_NetInvestmentIncomeOfEquityMethodInvestees / us-gaap_InvestmentTypeAxis = sigi_InvestmentsAccountedForUnderTheEquityMethodMember | 226,000,000 sigi_NetInvestmentIncomeOfEquityMethodInvestees / us-gaap_InvestmentTypeAxis = sigi_InvestmentsAccountedForUnderTheEquityMethodMember
Realized gains |  |  |  |  |  |  |  |  |  |  |  | 581,000,000 sigi_RealizedGainsLossesOfEquityMethodInvestees / us-gaap_InvestmentTypeAxis = sigi_InvestmentsAccountedForUnderTheEquityMethodMember | 913,000,000 sigi_RealizedGainsLossesOfEquityMethodInvestees / us-gaap_InvestmentTypeAxis = sigi_InvestmentsAccountedForUnderTheEquityMethodMember | 1,015,000,000 sigi_RealizedGainsLossesOfEquityMethodInvestees / us-gaap_InvestmentTypeAxis = sigi_InvestmentsAccountedForUnderTheEquityMethodMember
Net change in unrealized appreciation (depreciation) |  |  |  |  |  |  |  |  |  |  |  | 1,098,000,000 sigi_NetChangeInUnrealizedAppreciationDepreciationOfEquityMethodInvestees / us-gaap_InvestmentTypeAxis = sigi_InvestmentsAccountedForUnderTheEquityMethodMember | 382,000,000 sigi_NetChangeInUnrealizedAppreciationDepreciationOfEquityMethodInvestees / us-gaap_InvestmentTypeAxis = sigi_InvestmentsAccountedForUnderTheEquityMethodMember | (100,000,000)sigi_NetChangeInUnrealizedAppreciationDepreciationOfEquityMethodInvestees / us-gaap_InvestmentTypeAxis = sigi_InvestmentsAccountedForUnderTheEquityMethodMember
Net Income |  |  |  |  |  |  |  |  |  |  |  | 1,905,000,000 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationNetIncomeLoss / us-gaap_InvestmentTypeAxis = sigi_InvestmentsAccountedForUnderTheEquityMethodMember | 1,701,000,000 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationNetIncomeLoss / us-gaap_InvestmentTypeAxis = sigi_InvestmentsAccountedForUnderTheEquityMethodMember | 1,141,000,000 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationNetIncomeLoss / us-gaap_InvestmentTypeAxis = sigi_InvestmentsAccountedForUnderTheEquityMethodMember
Partnership Interest [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net investment income earned |  |  |  |  |  |  |  |  | $ 13,600,000 us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember | $ 15,200,000 us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember | $ 9,000,000 us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_PartnershipInterestMember |  |  |

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