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Investments (Aggregated Balance Sheet Summarized Financial Information for Partnerhips in our Alternative Investment Portfolio) (Details) (Details) (Investments Accounted For Under The Equity Method [Member], USD $)
In Millions, unless otherwise specified
Sep. 30, 2014
Sep. 30, 2013
Investments Accounted For Under The Equity Method [Member]
    
Investments $ 10,096sigi_EquityMethodInvestmentsSummarizedFinancialInformationInvestments
/ us-gaap_InvestmentTypeAxis
= sigi_InvestmentsAccountedForUnderTheEquityMethodMember		 $ 11,020sigi_EquityMethodInvestmentsSummarizedFinancialInformationInvestments
/ us-gaap_InvestmentTypeAxis
= sigi_InvestmentsAccountedForUnderTheEquityMethodMember
Total assets 10,695us-gaap_EquityMethodInvestmentSummarizedFinancialInformationAssets
/ us-gaap_InvestmentTypeAxis
= sigi_InvestmentsAccountedForUnderTheEquityMethodMember		 11,727us-gaap_EquityMethodInvestmentSummarizedFinancialInformationAssets
/ us-gaap_InvestmentTypeAxis
= sigi_InvestmentsAccountedForUnderTheEquityMethodMember
Total liabilities 545us-gaap_EquityMethodInvestmentSummarizedFinancialInformationLiabilities
/ us-gaap_InvestmentTypeAxis
= sigi_InvestmentsAccountedForUnderTheEquityMethodMember		 573us-gaap_EquityMethodInvestmentSummarizedFinancialInformationLiabilities
/ us-gaap_InvestmentTypeAxis
= sigi_InvestmentsAccountedForUnderTheEquityMethodMember
Partners' capital $ 10,150us-gaap_EquityMethodInvestmentSummarizedFinancialInformationEquity
/ us-gaap_InvestmentTypeAxis
= sigi_InvestmentsAccountedForUnderTheEquityMethodMember		 $ 11,154us-gaap_EquityMethodInvestmentSummarizedFinancialInformationEquity
/ us-gaap_InvestmentTypeAxis
= sigi_InvestmentsAccountedForUnderTheEquityMethodMember