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Investments (Components Of Net Realized Gains, Excluding OTTI Charges) (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
HTM fixed income securities Gains	$ 0sigi_HeldToMaturitySecuritiesRealizedGain	$ 0sigi_HeldToMaturitySecuritiesRealizedGain
HTM fixed income securities Losses	(1,000)sigi_HeldToMaturitySecuritiesDisposalSecurityRealizedLoss	(11,000)sigi_HeldToMaturitySecuritiesDisposalSecurityRealizedLoss
Other investments Gains	0sigi_OtherInvestmentsRealizedGains	0sigi_OtherInvestmentsRealizedGains
Other Investments Losses	(654,000)sigi_OtherInvestmentsRealizedLosses	0sigi_OtherInvestmentsRealizedLosses
Total other net realized investment gains	20,977,000us-gaap_GainLossOnSaleOfSecuritiesNet	8,181,000us-gaap_GainLossOnSaleOfSecuritiesNet
Proceeds From Sale Of Available For Sale Securities	138,400,000us-gaap_ProceedsFromSaleOfAvailableForSaleSecurities	62,800,000us-gaap_ProceedsFromSaleOfAvailableForSaleSecurities
Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
AFS securities Gains	1,502,000us-gaap_AvailableForSaleSecuritiesGrossRealizedGains / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember	158,000us-gaap_AvailableForSaleSecuritiesGrossRealizedGains / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember
AFS securities Losses	(112,000)us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember	(112,000)us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
AFS securities Gains	21,318,000us-gaap_AvailableForSaleSecuritiesGrossRealizedGains / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember	8,317,000us-gaap_AvailableForSaleSecuritiesGrossRealizedGains / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
AFS securities Losses	$ (1,076,000)us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember	$ (171,000)us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember

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This item represents the gross loss realized on the disposal of held-to-maturity securities during the period.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

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