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Allowance for Loan Losses - Accrued Interest Receivable (Detail) - USD ($)
$ in Millions
Jun. 30, 2016
Dec. 31, 2015
Private Education Loans - Traditional [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Accrued Interest Receivable $ 385 $ 433
Allowance for Uncollectible Interest 25 26
Private Education Loans - Non-Traditional [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Accrued Interest Receivable 49 57
Allowance for Uncollectible Interest 8 9
Private Education Loans [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Accrued Interest Receivable 434 490
Allowance for Uncollectible Interest $ 33 $ 35