Allowance for Loan Losses - Allowance for Credit Losses and Recorded Investments in Loans (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Allowance for Loan Losses
Beginning balance	$ 744	$ 920	$ 832	$ 1,022
Provision:
Total provision	14	11	26	(3)
Charge-offs:
Gross charge-offs	(87)	(92)	(207)	(198)
Expected future recoveries on current period gross charge-offs	10	11	21	24
Net charge-offs	(77)	(81)	(186)	(174)
Decrease in expected future recoveries on previously fully charged-off loans	6	7	15	12
Allowance at end of period	687	857	687	857
FFELP Loans [Member]
Allowance for Loan Losses
Beginning balance	206	214	215	222
Provision:
Total provision	(2)	5	(1)	15
Charge-offs:
Gross charge-offs	(10)	(19)	(20)	(37)
Expected future recoveries on current period gross charge-offs	0	0	0	0
Net charge-offs	(10)	(19)	(20)	(37)
Decrease in expected future recoveries on previously fully charged-off loans	0	0	0	0
Allowance at end of period	$ 194	$ 200	$ 194	$ 200
Net charge-offs as a percentage of average loans in repayment (annualized)	0.14%	0.22%	0.14%	0.22%
Ending total loans	$ 33,134	$ 41,051	$ 33,134	$ 41,051
Average loans in repayment	27,509	33,790	28,622	34,046
Ending loans in repayment	26,411	33,076	26,411	33,076
Private Education Loans [Member]
Allowance for Loan Losses
Beginning balance	538	706	617	800
Provision:
Total provision	16	6	27	(18)
Charge-offs:
Gross charge-offs	(77)	(73)	(187)	(161)
Expected future recoveries on current period gross charge-offs	10	11	21	24
Net charge-offs	(67)	(62)	(166)	(137)
Decrease in expected future recoveries on previously fully charged-off loans	6	7	15	12
Allowance at end of period	$ 493	$ 657	$ 493	$ 657
Net charge-offs as a percentage of average loans in repayment (annualized)	1.65%	1.39%	2.03%	1.51%
Ending total loans	$ 16,731	$ 18,389	$ 16,731	$ 18,389
Average loans in repayment	16,271	17,990	16,471	18,270
Ending loans in repayment	$ 16,087	$ 17,720	$ 16,087	$ 17,720