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Allowance for Loan Losses - Accrued Interest Receivable (Detail) - USD ($)
$ in Millions
Mar. 31, 2017
Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Accrued Interest Receivable $ 1,604 $ 1,663
Private Education Loans [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Accrued Interest Receivable 371 391
Accrued Interest Receivable, Allowance for Uncollectible Interest 26 30
Private Education Loans [Member] | Traditional Loan [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Accrued Interest Receivable 326 344
Accrued Interest Receivable, Allowance for Uncollectible Interest 20 23
Private Education Loans [Member] | Non Traditional Loan [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Accrued Interest Receivable 45 47
Accrued Interest Receivable, Allowance for Uncollectible Interest 6 7
Accrued Interest Receivable, Greater Than 90 Days Past Due [Member] | Private Education Loans [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Accrued Interest Receivable 30 33
Accrued Interest Receivable, Greater Than 90 Days Past Due [Member] | Private Education Loans [Member] | Traditional Loan [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Accrued Interest Receivable 23 26
Accrued Interest Receivable, Greater Than 90 Days Past Due [Member] | Private Education Loans [Member] | Non Traditional Loan [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Accrued Interest Receivable $ 7 $ 7