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Federal Income Taxes Federal Income Tax (Federal Tax Net Operating Loss Carryforwards) (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Operating Loss Carryforwards [Line Items]  
Annual Limitation Of Net Operating Loss Carryforwards $ 1.9sigi_AnnualLimitationOfNetOperatingLossCarryforwards
Operating Loss Carryforwards 6,102us-gaap_OperatingLossCarryforwards
Operating Loss Carryforwards Tax Effect 2,136sigi_OperatingLossCarryforwardsTaxEffect
Tax Year 2029 [Member]  
Operating Loss Carryforwards [Line Items]  
Operating Loss Carryforwards 2,024us-gaap_OperatingLossCarryforwards
/ us-gaap_TaxPeriodAxis
= sigi_TaxYear2029Member
Operating Loss Carryforwards Tax Effect 708sigi_OperatingLossCarryforwardsTaxEffect
/ us-gaap_TaxPeriodAxis
= sigi_TaxYear2029Member
Tax Year 2030 [Member] [Member]  
Operating Loss Carryforwards [Line Items]  
Operating Loss Carryforwards 3,999us-gaap_OperatingLossCarryforwards
/ us-gaap_TaxPeriodAxis
= sigi_TaxYear2030MemberMember
Operating Loss Carryforwards Tax Effect 1,400sigi_OperatingLossCarryforwardsTaxEffect
/ us-gaap_TaxPeriodAxis
= sigi_TaxYear2030MemberMember
Tax Year 2031 [Member] [Member]  
Operating Loss Carryforwards [Line Items]  
Operating Loss Carryforwards 79us-gaap_OperatingLossCarryforwards
/ us-gaap_TaxPeriodAxis
= sigi_TaxYear2031MemberMember
Operating Loss Carryforwards Tax Effect $ 28sigi_OperatingLossCarryforwardsTaxEffect
/ us-gaap_TaxPeriodAxis
= sigi_TaxYear2031MemberMember