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Allowance for Loan Losses - Receivable for Partially Charged-Off Loans (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Financing Receivable, Allowance for Credit Losses [Line Items]
Net adjustment resulting from the change in the charge-off rate		$ (330)		$ (330)
Private Education Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Receivable at beginning of period	$ 867	1,236	$ 881	1,245
Expected future recoveries of current period defaults	32	46	68	108
Recoveries	(52)	(50)	(102)	(102)
Net adjustment resulting from the change in the charge-off rate		(330)		(330)
Net charge-offs remaining				(19)
Total net charge-offs		(330)		(349)
Receivable at end of period	$ 847	$ 902	$ 847	$ 902