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Investments (Performing And Nonperforming Commercial Mortgage Loans By Property Type) (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Financing Receivable, Recorded Investment [Line Items]    
Commercial mortgage loans, net $ 5,321.1us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet $ 5,405.1us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
Performing Financing Receivable [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Commercial mortgage loans, net 5,314.9us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PerformingFinancingReceivableMember		 5,393.2us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PerformingFinancingReceivableMember
Nonperforming Financing Receivable [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Commercial mortgage loans, net 6.2us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_NonperformingFinancingReceivableMember		 11.9us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_NonperformingFinancingReceivableMember
Retail [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Commercial mortgage loans, net 2,627.1us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis
= sfg_RetailMember		 2,629.2us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis
= sfg_RetailMember
Retail [Member] | Performing Financing Receivable [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Commercial mortgage loans, net 2,622.4us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis
= sfg_RetailMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PerformingFinancingReceivableMember		 2,624.2us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis
= sfg_RetailMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PerformingFinancingReceivableMember
Retail [Member] | Nonperforming Financing Receivable [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Commercial mortgage loans, net 4.7us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis
= sfg_RetailMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_NonperformingFinancingReceivableMember		 5.0us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis
= sfg_RetailMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_NonperformingFinancingReceivableMember
Office [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Commercial mortgage loans, net 1,013.4us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis
= sfg_OfficeMember		 982.4us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis
= sfg_OfficeMember
Office [Member] | Performing Financing Receivable [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Commercial mortgage loans, net 1,012.7us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis
= sfg_OfficeMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PerformingFinancingReceivableMember		 979.6us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis
= sfg_OfficeMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PerformingFinancingReceivableMember
Office [Member] | Nonperforming Financing Receivable [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Commercial mortgage loans, net 0.7us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis
= sfg_OfficeMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_NonperformingFinancingReceivableMember		 2.8us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis
= sfg_OfficeMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_NonperformingFinancingReceivableMember
Industrial [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Commercial mortgage loans, net 1,001.5us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis
= sfg_IndustrialMember		 1,008.7us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis
= sfg_IndustrialMember
Industrial [Member] | Performing Financing Receivable [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Commercial mortgage loans, net 1,001.4us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis
= sfg_IndustrialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PerformingFinancingReceivableMember		 1,004.6us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis
= sfg_IndustrialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PerformingFinancingReceivableMember
Industrial [Member] | Nonperforming Financing Receivable [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Commercial mortgage loans, net 0.1us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis
= sfg_IndustrialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_NonperformingFinancingReceivableMember		 4.1us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis
= sfg_IndustrialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_NonperformingFinancingReceivableMember
Hotels And Motels [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Commercial mortgage loans, net 165.7us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis
= sfg_HotelsAndMotelsMember		 276.3us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis
= sfg_HotelsAndMotelsMember
Hotels And Motels [Member] | Performing Financing Receivable [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Commercial mortgage loans, net 165.7us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis
= sfg_HotelsAndMotelsMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PerformingFinancingReceivableMember		 276.3us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis
= sfg_HotelsAndMotelsMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PerformingFinancingReceivableMember
Hotels And Motels [Member] | Nonperforming Financing Receivable [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Commercial mortgage loans, net 0us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis
= sfg_HotelsAndMotelsMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_NonperformingFinancingReceivableMember		 0us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis
= sfg_HotelsAndMotelsMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_NonperformingFinancingReceivableMember
Commercial [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Commercial mortgage loans, net 213.6us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis
= sfg_CommercialMember		 218.6us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis
= sfg_CommercialMember
Commercial [Member] | Performing Financing Receivable [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Commercial mortgage loans, net 213.6us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis
= sfg_CommercialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PerformingFinancingReceivableMember		 218.6us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis
= sfg_CommercialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PerformingFinancingReceivableMember
Commercial [Member] | Nonperforming Financing Receivable [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Commercial mortgage loans, net 0us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis
= sfg_CommercialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_NonperformingFinancingReceivableMember		 0us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis
= sfg_CommercialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_NonperformingFinancingReceivableMember
Apartment And Other [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Commercial mortgage loans, net 299.8us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis
= sfg_ApartmentAndOtherMember		 289.9us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis
= sfg_ApartmentAndOtherMember
Apartment And Other [Member] | Performing Financing Receivable [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Commercial mortgage loans, net 299.1us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis
= sfg_ApartmentAndOtherMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PerformingFinancingReceivableMember		 289.9us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis
= sfg_ApartmentAndOtherMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PerformingFinancingReceivableMember
Apartment And Other [Member] | Nonperforming Financing Receivable [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Commercial mortgage loans, net $ 0.7us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis
= sfg_ApartmentAndOtherMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_NonperformingFinancingReceivableMember		 $ 0us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis
= sfg_ApartmentAndOtherMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_NonperformingFinancingReceivableMember