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Loans Held for Investment - Student Loan Portfolio by Program (Detail) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2017
Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Allowance for loan losses $ (272,123) $ (251,475) $ (187,086) $ (184,701)
Loans held for investment, net 20,166,604 18,567,641    
Private Education Loans        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Private Education Loans 18,794,012 17,432,167    
Deferred origination costs and unamortized premium/(discount) 58,814 56,378    
Allowance for loan losses (252,103) (243,715)    
Loans held for investment, net 18,600,723 17,244,830    
FFELP Loans        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Private Education Loans 907,842 927,660    
Deferred origination costs and unamortized premium/(discount) 2,566 2,631    
Allowance for loan losses (1,113) (1,132)    
Loans held for investment, net 909,295 929,159    
Personal Loans        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Private Education Loans 675,656 400,280    
Deferred origination costs and unamortized premium/(discount) (163) 0    
Allowance for loan losses (18,907) (6,628)    
Loans held for investment, net $ 656,586 $ 393,652