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Income Taxes (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2012
Dec. 31, 2013
Operating Loss Carryforwards [Line Items]      
Federal corporate tax rate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate     
Carrying forward net operating losses $ 54.0us-gaap_OperatingLossCarryforwards    
Valuation Allowance, Other Assets 1.1sfg_Operatinglosscarryforwardsvaluationallowanceotherassets    
Valuation Allowance 8.4us-gaap_OperatingLossCarryforwardsValuationAllowance    
Deferred tax assets, valuation allowance 9.5us-gaap_DeferredTaxAssetsValuationAllowance 3.1us-gaap_DeferredTaxAssetsValuationAllowance 9.5us-gaap_DeferredTaxAssetsValuationAllowance
Tax years open for examination the Company is subject to examination by the IRS for the years 2011 through 2014    
Maximum [Member]      
Operating Loss Carryforwards [Line Items]      
Unutilized NOLs expiration date Dec. 31, 2033    
Minimum [Member]      
Operating Loss Carryforwards [Line Items]      
Unutilized NOLs expiration date Dec. 31, 2023    
State and municipalities [Member]      
Operating Loss Carryforwards [Line Items]      
Carrying forward a deferred tax asset 13.2us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsSubjectToExpiration
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember
   
State and municipalities [Member] | Maximum [Member]      
Operating Loss Carryforwards [Line Items]      
Unutilized NOLs expiration date Dec. 31, 2033    
Prior Business Combinations [Member]      
Operating Loss Carryforwards [Line Items]      
Carrying forward net operating losses $ 46.0us-gaap_OperatingLossCarryforwards
/ us-gaap_BusinessAcquisitionAxis
= sfg_CollectiveBusinessCombinationMember
   
Prior Business Combinations [Member] | Maximum [Member]      
Operating Loss Carryforwards [Line Items]      
Unutilized NOLs expiration date Dec. 31, 2025    
Prior Business Combinations [Member] | Minimum [Member]      
Operating Loss Carryforwards [Line Items]      
Unutilized NOLs expiration date Dec. 31, 2020