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Allowance for Loan Losses - Additional Information (Detail)
$ in Millions
 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2015
USD ($)
Mar. 31, 2015
Jun. 30, 2016
USD ($)
Point
Loans
Jun. 30, 2015
USD ($)
Dec. 31, 2015
USD ($)
Financing Receivable, Allowance for Credit Losses [Line Items]          
Maximum original FICO score at for-profit schools to become eligible for non-traditional loans | Point     670    
Maximum original FICO score at not-for-profit schools to become eligible for non-traditional loans | Point     640    
Net adjustment resulting from the change in the charge-off rate $ 330     $ 330  
Percentage of loans granted forbearance that migrated to TDR classification     58.00%   56.00%
Payment default period for TDRs     60 days past due    
Interest Rate Reduction [Member]          
Financing Receivable, Allowance for Credit Losses [Line Items]          
TDR loans, Unpaid Principal Balance     $ 2,800   $ 2,500
Minimum [Member]          
Financing Receivable, Allowance for Credit Losses [Line Items]          
Delinquency period (in days)         212 days
Private Education Loans [Member]          
Financing Receivable, Allowance for Credit Losses [Line Items]          
Number of classes of loans | Loans     2    
Portion of loan amount charged off at default 79.00% 73.00%      
Net adjustment resulting from the change in the charge-off rate $ 330     $ 330  
Loans default recovery rate 21.00%