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Investments Investments - Maturity Table (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]    
Investments at cost $ 167,740us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis $ 106,977us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
Mortgage-backed securities 168,934us-gaap_AvailableForSaleSecuritiesDebtSecurities 102,105us-gaap_AvailableForSaleSecuritiesDebtSecurities
Maturity 2038    
Schedule of Available-for-sale Securities [Line Items]    
Investments at cost 604us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_DebtSecurityAxis
= slm_Maturity2038Member		  
Mortgage-backed securities 647us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_DebtSecurityAxis
= slm_Maturity2038Member		  
Maturity 2039    
Schedule of Available-for-sale Securities [Line Items]    
Investments at cost 11,298us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_DebtSecurityAxis
= slm_Maturity2039Member		  
Mortgage-backed securities 12,145us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_DebtSecurityAxis
= slm_Maturity2039Member		  
Maturity 2042    
Schedule of Available-for-sale Securities [Line Items]    
Investments at cost 27,721us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_DebtSecurityAxis
= slm_Maturity2042Member		  
Mortgage-backed securities 26,617us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_DebtSecurityAxis
= slm_Maturity2042Member		  
Maturity 2043    
Schedule of Available-for-sale Securities [Line Items]    
Investments at cost 73,534us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_DebtSecurityAxis
= slm_Maturity2043Member		  
Mortgage-backed securities 74,505us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_DebtSecurityAxis
= slm_Maturity2043Member		  
Maturity 2044    
Schedule of Available-for-sale Securities [Line Items]    
Investments at cost 54,583us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_DebtSecurityAxis
= slm_Maturity2044Member		  
Mortgage-backed securities $ 55,020us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_DebtSecurityAxis
= slm_Maturity2044Member