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Allowance for Loan Losses - Accrued Interest Receivable (Detail) - USD ($)
$ in Millions
Sep. 30, 2016
Dec. 31, 2015
Private Education Loans - Traditional [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Accrued Interest Receivable $ 374 $ 433
Greater Than 90 Days Past Due 23 27
Allowance for Uncollectible Interest 26 26
Private Education Loans - Non-Traditional [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Accrued Interest Receivable 48 57
Greater Than 90 Days Past Due 7 8
Allowance for Uncollectible Interest 7 9
Private Education Loans [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Accrued Interest Receivable 422 490
Greater Than 90 Days Past Due 30 35
Allowance for Uncollectible Interest $ 33 $ 35