Allowance for Loan Losses - Allowance for Credit Losses and Recorded Investments in Loans (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Allowance for Loan Losses
Beginning balance	$ 857	$ 1,166	$ 1,022	$ 1,271
Provision:
Total provision	72	28	68	62
Charge-offs:
Gross charge-offs	(101)	130	(298)	310
Expected future recoveries on current period gross charge-offs	12	19	36	43
Total	(89)	(111)	(262)	(267)
Adjustment resulting from the change in charge-off rate	(25)	30	(25)	30
Net charge-offs	(114)	(141)	(287)	(297)
Decrease in expected future recoveries on previously fully charged-off loans	30	32	42	49
Allowance at end of period	845	1,085	845	1,085
FFELP Loans [Member]
Allowance for Loan Losses
Beginning balance	200	245	222	262
Provision:
Total provision	36	0	51	0
Charge-offs:
Gross charge-offs	(16)	12	(53)	29
Expected future recoveries on current period gross charge-offs	0	0	0	0
Total	(16)	(12)	(53)	(29)
Adjustment resulting from the change in charge-off rate	0	0	0
Net charge-offs	(16)	(12)	(53)	(29)
Decrease in expected future recoveries on previously fully charged-off loans	0	0	0	0
Allowance at end of period	$ 220	$ 233	$ 220	$ 233
Net charge-offs as a percentage of average loans in repayment, excluding the net adjustment resulting from the change in charge-off rate (annualized)	0.19%	0.12%	0.21%	0.09%
Net adjustment resulting from the change in charge-off rate as a percentage of average loans in repayment (annualized)	0.00%	0.00%	0.00%	0.00%
Net charge-offs as a percentage of average loans in repayment (annualized)	0.19%	0.12%	0.21%	0.09%
Ending total loans	$ 39,801	$ 47,124	$ 39,801	$ 47,124
Average loans in repayment	32,696	39,573	33,591	41,793
Ending loans in repayment	31,917	37,731	31,917	37,731
Private Education Loans [Member]
Allowance for Loan Losses
Beginning balance	657	921	800	1,009
Provision:
Total provision	36	28	17	62
Charge-offs:
Gross charge-offs	(85)	118	(245)	281
Expected future recoveries on current period gross charge-offs	12	19	36	43
Total	(73)	(99)	(209)	(238)
Adjustment resulting from the change in charge-off rate	(25)	30	(25)	30
Net charge-offs	(98)	(129)	(234)	(268)
Decrease in expected future recoveries on previously fully charged-off loans	30	32	42	49
Allowance at end of period	$ 625	$ 852	$ 625	$ 852
Net charge-offs as a percentage of average loans in repayment, excluding the net adjustment resulting from the change in charge-off rate (annualized)	1.66%	2.01%	0.00%	1.59%
Net adjustment resulting from the change in charge-off rate as a percentage of average loans in repayment (annualized)	0.56%	0.60%	0.18%	0.20%
Net charge-offs as a percentage of average loans in repayment (annualized)	2.22%	2.61%	1.74%	1.79%
Ending total loans	$ 17,958	$ 20,003	$ 17,958	$ 20,003
Average loans in repayment	17,470	19,628	18,000	20,056
Ending loans in repayment	$ 17,249	$ 19,284	$ 17,249	$ 19,284