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Investments Investments - Additional Information (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Oct. 31, 2013
Investment Holdings [Line Items]      
Investments at cost $ 167,740us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis $ 106,977us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis  
Gain on sale of asset-backed security     63,813slm_Gainonsaleofassetbackedsecurity
Par value of mortgage-backed securities pledged to FRB 160,949slm_ParValueofMortgagebackedsecuritiespledgedtoFRB 103,049slm_ParValueofMortgagebackedsecuritiespledgedtoFRB  
Mortgage-backed securities      
Investment Holdings [Line Items]      
Investments at cost 167,740us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesMember		 106,977us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesMember		  
Number of mortgage-backed securities with unrealized losses 10slm_NumberofMortgageBackedsecuritieswithunrealizedlosses
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesMember		 20slm_NumberofMortgageBackedsecuritieswithunrealizedlosses
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesMember		  
Number of mortgage-backed securities 13slm_NumberofMortgagebackedsecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesMember		 33slm_NumberofMortgagebackedsecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesMember		  
Ginnie Mae | Mortgage-backed securities      
Investment Holdings [Line Items]      
Investments at cost 86,329us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ invest_InvestmentIssuerAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesMember		    
Number of mortgage-backed securities with unrealized losses 9slm_NumberofMortgageBackedsecuritieswithunrealizedlosses
/ invest_InvestmentIssuerAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesMember		 10slm_NumberofMortgageBackedsecuritieswithunrealizedlosses
/ invest_InvestmentIssuerAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesMember		  
Fannie Mae | Mortgage-backed securities      
Investment Holdings [Line Items]      
Investments at cost 65,850us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ invest_InvestmentIssuerAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesMember		    
Freddie Mac | Mortgage-backed securities      
Investment Holdings [Line Items]      
Investments at cost $ 15,561us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ invest_InvestmentIssuerAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesMember