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Allowance for Loan Losses - Additional Information (Detail)
$ in Billions
 3 Months Ended 12 Months Ended
Mar. 31, 2017
USD ($)
Point
Loans
Dec. 31, 2016
USD ($)
Financing Receivable, Allowance for Credit Losses [Line Items]    
Percentage of loans granted forbearance that migrated to TDR classification 62.00% 61.00%
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 | $ $ 2.5 $ 2.6
Minimum [Member]    
Financing Receivable, Allowance for Credit Losses [Line Items]    
Delinquency period (in days) 212 days  
Minimum [Member] | FICO Score, 600 to 699 [Member]    
Financing Receivable, Allowance for Credit Losses [Line Items]    
Original FICO score at for-profit schools to become eligible for non-traditional loans 640  
Maximum [Member] | FICO Score, 600 to 699 [Member]    
Financing Receivable, Allowance for Credit Losses [Line Items]    
Original FICO score at for-profit schools to become eligible for non-traditional loans 670  
Private Education Loans [Member]    
Financing Receivable, Allowance for Credit Losses [Line Items]    
Number of classes of loans | Loans 2