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Investments (Unrealized Gains on Investments) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Net unrealized gains	$ 123,683sigi_UnrealizedGainLossOnInvestmentSecurities	$ 79,237sigi_UnrealizedGainLossOnInvestmentSecurities	$ 188,197sigi_UnrealizedGainLossOnInvestmentSecurities
Deferred income tax expense	(43,289)sigi_DeferredIncomeTaxExpenseBenefitOnInvestmentSecuritiesInOtherComprehensiveIncome	(27,733)sigi_DeferredIncomeTaxExpenseBenefitOnInvestmentSecuritiesInOtherComprehensiveIncome	(65,869)sigi_DeferredIncomeTaxExpenseBenefitOnInvestmentSecuritiesInOtherComprehensiveIncome
Net unrealized gains, net of deferred income tax	80,394sigi_Netunrealizedgainlossesoninvestmentsecuritiesinothercomprehensiveincomenetofdeferredincometax	51,504sigi_Netunrealizedgainlossesoninvestmentsecuritiesinothercomprehensiveincomenetofdeferredincometax	122,328sigi_Netunrealizedgainlossesoninvestmentsecuritiesinothercomprehensiveincomenetofdeferredincometax
Increase (Decrease) in net unrealized gains in OCI, net of deferred income tax	28,890sigi_TotalUnrealizedGainsLossesonInvestmentSecurities	(70,824)sigi_TotalUnrealizedGainsLossesonInvestmentSecurities	25,080sigi_TotalUnrealizedGainsLossesonInvestmentSecurities
AFS Fixed Income Securities [Member]
Net unrealized gains	90,336sigi_UnrealizedGainLossOnInvestmentSecurities / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember	39,559sigi_UnrealizedGainLossOnInvestmentSecurities / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember	165,330sigi_UnrealizedGainLossOnInvestmentSecurities / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember
Equity Securities [Member]
Net unrealized gains	32,389sigi_UnrealizedGainLossOnInvestmentSecurities / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember	37,421sigi_UnrealizedGainLossOnInvestmentSecurities / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember	18,941sigi_UnrealizedGainLossOnInvestmentSecurities / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember
Available-for-sale Securities [Member]
Net unrealized gains	122,725sigi_UnrealizedGainLossOnInvestmentSecurities / us-gaap_InvestmentTypeAxis = us-gaap_AvailableforsaleSecuritiesMember	76,980sigi_UnrealizedGainLossOnInvestmentSecurities / us-gaap_InvestmentTypeAxis = us-gaap_AvailableforsaleSecuritiesMember	184,271sigi_UnrealizedGainLossOnInvestmentSecurities / us-gaap_InvestmentTypeAxis = us-gaap_AvailableforsaleSecuritiesMember
Total HTM Securities [Member]
Net unrealized gains	$ 958sigi_UnrealizedGainLossOnInvestmentSecurities / us-gaap_InvestmentTypeAxis = us-gaap_HeldtomaturitySecuritiesMember	$ 2,257sigi_UnrealizedGainLossOnInvestmentSecurities / us-gaap_InvestmentTypeAxis = us-gaap_HeldtomaturitySecuritiesMember	$ 3,926sigi_UnrealizedGainLossOnInvestmentSecurities / us-gaap_InvestmentTypeAxis = us-gaap_HeldtomaturitySecuritiesMember

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- Definition

Tax effect of unrealized gain (loss) on investments in accumulated other comprehensive income.

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