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Share-Based Compensation (Narrative-Options) (Details) (Stock Incentive 2002 Plan [Member], USD $)
In Millions, except Share data, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Number of shares of common stock authorized for share based compensation arrangements 7,000,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized  
Number of shares of common stock available for grant 2,445,466us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant  
Employee Stock Option [Member]
   
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Award vesting period 4 years 0 months 0 days  
Compensation cost related to unvested option, not recognized $ 7.5us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
/ us-gaap_PlanNameAxis
= sfg_StockIncentive2002PlanMember
 
Weighted average recognition period of unrecognized compensation cost for stock options 2 years 8 months 12 days  
Stock option awards granted in period 133,968us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
/ us-gaap_PlanNameAxis
= sfg_StockIncentive2002PlanMember
105,060us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
/ us-gaap_PlanNameAxis
= sfg_StockIncentive2002PlanMember
Weighted average exercise price of stock option awards granted in period $ 66.21us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
/ us-gaap_PlanNameAxis
= sfg_StockIncentive2002PlanMember
$ 63.74us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
/ us-gaap_PlanNameAxis
= sfg_StockIncentive2002PlanMember
Weighted average grant date fair value of stock option awards granted in period $ 18.83us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
/ us-gaap_PlanNameAxis
= sfg_StockIncentive2002PlanMember
$ 24.53us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
/ us-gaap_PlanNameAxis
= sfg_StockIncentive2002PlanMember