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Allowance for Loan Losses - Modified Loan Accounts for TDR (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Receivables [Abstract]  
Modified Loans $ 59,402slm_ModifiedLoans
Charge-offs 948slm_ChargeOffsInTDRPortfolio
Payment-Default $ 325slm_TroubledDebtRestructuringsPaymentDefault