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Allowance for Loan Losses - Receivable for Partially Charged-Off Loans (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Jun. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Financing Receivable, Allowance for Credit Losses [Line Items]          
Net adjustment resulting from the change in the charge-off rate         $ (330)
Net charge-offs remaining         (549)
Total net charge-offs $ (130) $ (161)   $ (434) 879
Private Education Loans [Member]          
Financing Receivable, Allowance for Credit Losses [Line Items]          
Receivable at beginning of period 847 902   881 1,245
Expected future recoveries of current period defaults 28 38   96 147
Recoveries (47) (48)   (149) (151)
Net adjustment resulting from the change in the charge-off rate     $ (330)   (330)
Net charge-offs remaining         (517)
Total net charge-offs (112) (148)   (383) 847
Receivable at end of period $ 828 $ 892 $ 902 $ 828 892
Private Education Loans [Member] | Loans Receivable For Partially Charged Off Loans [Member]          
Financing Receivable, Allowance for Credit Losses [Line Items]          
Net charge-offs remaining         (19)
Total net charge-offs         $ (349)