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Reinsurance (List of direct, assumed and ceded Reinsurance Amounts) (Details) (USD $)
 3 Months Ended 12 Months Ended
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2013
Sep. 30, 2013
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Premiums Written:                      
Direct                 $ 2,228,270,000us-gaap_DirectPremiumsWritten $ 2,133,793,000us-gaap_DirectPremiumsWritten $ 1,955,667,000us-gaap_DirectPremiumsWritten
Assumed                 26,306,000us-gaap_AssumedPremiumsWritten 43,650,000us-gaap_AssumedPremiumsWritten 50,938,000us-gaap_AssumedPremiumsWritten
Ceded premiums written                 (369,296,000)us-gaap_CededPremiumsWritten (367,284,000)us-gaap_CededPremiumsWritten (339,722,000)us-gaap_CededPremiumsWritten
Net                 1,885,280,000us-gaap_PremiumsWrittenNet 1,810,159,000us-gaap_PremiumsWrittenNet 1,666,883,000us-gaap_PremiumsWrittenNet
Premiums Earned:                      
Direct                 2,183,258,000us-gaap_DirectPremiumsEarned 2,048,530,000us-gaap_DirectPremiumsEarned 1,873,007,000us-gaap_DirectPremiumsEarned
Assumed                 34,653,000us-gaap_AssumedPremiumsEarned 44,464,000us-gaap_AssumedPremiumsEarned 65,884,000us-gaap_AssumedPremiumsEarned
Ceded premiums earned                 (365,302,000)us-gaap_CededPremiumsEarned (356,922,000)us-gaap_CededPremiumsEarned (354,772,000)us-gaap_CededPremiumsEarned
Total Net Premiums Earned 469,850,000us-gaap_PremiumsEarnedNetPropertyAndCasualty 462,639,000us-gaap_PremiumsEarnedNetPropertyAndCasualty 463,625,000us-gaap_PremiumsEarnedNetPropertyAndCasualty 456,495,000us-gaap_PremiumsEarnedNetPropertyAndCasualty 451,312,000us-gaap_PremiumsEarnedNetPropertyAndCasualty 437,568,000us-gaap_PremiumsEarnedNetPropertyAndCasualty 426,252,000us-gaap_PremiumsEarnedNetPropertyAndCasualty 420,940,000us-gaap_PremiumsEarnedNetPropertyAndCasualty 1,852,609,000us-gaap_PremiumsEarnedNetPropertyAndCasualty 1,736,072,000us-gaap_PremiumsEarnedNetPropertyAndCasualty 1,584,119,000us-gaap_PremiumsEarnedNetPropertyAndCasualty
Losses and loss expenes incurred:                      
Direct                 1,314,864,000us-gaap_PolicyholderBenefitsAndClaimsIncurredGross 1,370,293,000us-gaap_PolicyholderBenefitsAndClaimsIncurredGross 2,394,640,000us-gaap_PolicyholderBenefitsAndClaimsIncurredGross
Assumed                 26,187,000us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountAssumed 32,678,000us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountAssumed 29,175,000us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountAssumed
Ceded losses and loss expenses incurred                 (183,550,000)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded (281,233,000)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded (1,302,825,000)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded
Net                 1,157,501,000us-gaap_IncurredClaimsPropertyCasualtyAndLiability 1,121,738,000us-gaap_IncurredClaimsPropertyCasualtyAndLiability 1,120,990,000us-gaap_IncurredClaimsPropertyCasualtyAndLiability
Superstorm Sandy [Member]                      
Increase Decrease In Reinsurance Effect On Claims And Benefits Incurred Amount Ceded                     $ 1,100,000,000sigi_IncreaseDecreaseInReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded
/ us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis
= sigi_SuperstormSandyMember