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

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This item represents the disclosure of summarized financial information, specifically the aggregate change in unrealized appreciation (depreciation), for unconsolidated subsidiaries and 50 percent-or-less owned entities accounted for under the equity method of accounting.

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This item represents the disclosure of summarized financial information, specifically aggregate realized gains (losses), for unconsolidated subsidiaries and 50 percent-or-less owned entities accounted for under the equity method of accounting.

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The amount of net income (loss) reported by an equity method investment of the entity.

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Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.

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