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Allowance for Loan Losses - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Jul. 01, 2006
Receivables [Abstract]      
Troubled Debt Restructuring $ 122.1slm_TroubledDebtRestructuring    
Criteria for loans to be considered as nonperforming 90 days    
TDR payment default period 60 days    
Tier One of Government Guarantee 97.00%slm_TierOneofGovernmentGuarantee   97.00%slm_TierOneofGovernmentGuarantee
Ffelp Loans Net $ 741slm_FFELPRehabilitationLoans    
Percentage of FFELP Rehabilitation Loans 61.30%slm_PercentageofFFELPRehabilitationLoans 62.10%slm_PercentageofFFELPRehabilitationLoans  
Period of loans past due that have accrued interest 90 days