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Investments (Unrealized Gains on Investments) (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Net unrealized gains | $ 123,683 sigi_UnrealizedGainLossOnInvestmentSecurities | $ 79,237 sigi_UnrealizedGainLossOnInvestmentSecurities | $ 188,197 sigi_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 | 80,394 sigi_Netunrealizedgainlossesoninvestmentsecuritiesinothercomprehensiveincomenetofdeferredincometax | 51,504 sigi_Netunrealizedgainlossesoninvestmentsecuritiesinothercomprehensiveincomenetofdeferredincometax | 122,328 sigi_Netunrealizedgainlossesoninvestmentsecuritiesinothercomprehensiveincomenetofdeferredincometax
Increase (Decrease) in net unrealized gains in OCI, net of deferred income tax | 28,890 sigi_TotalUnrealizedGainsLossesonInvestmentSecurities | (70,824)sigi_TotalUnrealizedGainsLossesonInvestmentSecurities | 25,080 sigi_TotalUnrealizedGainsLossesonInvestmentSecurities
AFS Fixed Income Securities [Member] |  |  |
Net unrealized gains | 90,336 sigi_UnrealizedGainLossOnInvestmentSecurities / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember | 39,559 sigi_UnrealizedGainLossOnInvestmentSecurities / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember | 165,330 sigi_UnrealizedGainLossOnInvestmentSecurities / us-gaap_InvestmentTypeAxis = sigi_AfsFixedMaturitySecuritiesMember
Equity Securities [Member] |  |  |
Net unrealized gains | 32,389 sigi_UnrealizedGainLossOnInvestmentSecurities / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember | 37,421 sigi_UnrealizedGainLossOnInvestmentSecurities / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember | 18,941 sigi_UnrealizedGainLossOnInvestmentSecurities / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember
Available-for-sale Securities [Member] |  |  |
Net unrealized gains | 122,725 sigi_UnrealizedGainLossOnInvestmentSecurities / us-gaap_InvestmentTypeAxis = us-gaap_AvailableforsaleSecuritiesMember | 76,980 sigi_UnrealizedGainLossOnInvestmentSecurities / us-gaap_InvestmentTypeAxis = us-gaap_AvailableforsaleSecuritiesMember | 184,271 sigi_UnrealizedGainLossOnInvestmentSecurities / us-gaap_InvestmentTypeAxis = us-gaap_AvailableforsaleSecuritiesMember
Total HTM Securities [Member] |  |  |
Net unrealized gains | $ 958 sigi_UnrealizedGainLossOnInvestmentSecurities / us-gaap_InvestmentTypeAxis = us-gaap_HeldtomaturitySecuritiesMember | $ 2,257 sigi_UnrealizedGainLossOnInvestmentSecurities / us-gaap_InvestmentTypeAxis = us-gaap_HeldtomaturitySecuritiesMember | $ 3,926 sigi_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. + References No definition available. + Details
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- Definition This item represents the net unrealized gains (losses) on investments in other comprehensive income, net of deferred income tax. + References No definition available. + Details
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