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Federal Income Taxes (Deferred Tax Assets and Liabilities) (Details) (USD $)
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Net loss reserve discounting $ 84,502,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsLossReserves $ 87,967,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsLossReserves  
Net unearned premiums 66,470,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsUnearnedPremiumsReserve 64,167,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsUnearnedPremiumsReserve  
Employee benefits 33,721,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsEmployeeBenefits 19,912,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsEmployeeBenefits  
Long-term incentive compensation plans 13,625,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost 12,904,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost  
Temporary investment write-downs 3,939,000sigi_DeferredTaxAssetsUnrealizedLossesOnSecuritiesGross 7,586,000sigi_DeferredTaxAssetsUnrealizedLossesOnSecuritiesGross  
Net operating loss 2,136,000us-gaap_DeferredTaxAssetsOperatingLossCarryforwards 2,818,000us-gaap_DeferredTaxAssetsOperatingLossCarryforwards  
Tax credits 7,826,000sigi_DeferredTaxAssetTaxCredits 17,042,000sigi_DeferredTaxAssetTaxCredits  
Other 8,811,000us-gaap_DeferredTaxAssetsOther 10,088,000us-gaap_DeferredTaxAssetsOther  
Total deferred tax assets 221,030,000us-gaap_DeferredTaxAssetsGross 222,484,000us-gaap_DeferredTaxAssetsGross  
Deferred policy acquisition costs 63,242,000us-gaap_DeferredTaxLiabilitiesDeferredExpenseDeferredPolicyAcquisitionCost 59,164,000us-gaap_DeferredTaxLiabilitiesDeferredExpenseDeferredPolicyAcquisitionCost  
Unrealized gains on investment securities 43,289,000sigi_DeferredTaxLiabilitiesInvestmentSecuritiesUnrealizedGain 31,345,000sigi_DeferredTaxLiabilitiesInvestmentSecuritiesUnrealizedGain  
Deferred Tax Liabilities, Other 5,088,000us-gaap_DeferredTaxLiabilitiesOther 618,000us-gaap_DeferredTaxLiabilitiesOther  
Accelerated depreciation and amortization 10,962,000us-gaap_DeferredTaxLiabilitiesPropertyPlantAndEquipment 8,744,000us-gaap_DeferredTaxLiabilitiesPropertyPlantAndEquipment  
Total deferred tax liabilities 122,581,000us-gaap_DeferredTaxLiabilities 99,871,000us-gaap_DeferredTaxLiabilities  
Net deferred federal income tax asset 98,449,000us-gaap_DeferredTaxAssetsNet 122,613,000us-gaap_DeferredTaxAssetsNet  
Tax benefits related to compensation expense deductions for share-based compensation awards $ 20,200,000sigi_TaxBenefitsRelatedToCompensationExpenseDeductionsForShareBasedCompensationAwards $ 19,200,000sigi_TaxBenefitsRelatedToCompensationExpenseDeductionsForShareBasedCompensationAwards $ 17,700,000sigi_TaxBenefitsRelatedToCompensationExpenseDeductionsForShareBasedCompensationAwards