Allowance for Loan Losses - Allowance for Credit Losses and Recorded Investments in Loans (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Allowance for Loan Losses
Allowance at beginning of period	$ 1,022	$ 1,271	$ 1,377
Provision:
Reversal of allowance related to loan sales			(107)
Remaining provision			46
Total provision	123	79	(61)
Charge-offs:
Gross charge-offs	(383)	(410)	(201)
Expected future recoveries on current period gross charge-offs	47	57	22
Total	(336)	(353)	(179)
Adjustment resulting from the change in charge-off rate	(25)	(30)	(16)
Net charge-offs	(361)	(383)	(195)
Decrease in expected future recoveries on previously fully charged-off loans	48	55	150
Allowance at end of period	832	1,022	1,271
FFELP Loans [Member]
Allowance for Loan Losses
Allowance at beginning of period	222	262	288
Provision:
Reversal of allowance related to loan sales			0
Remaining provision			0
Total provision	56	0	0
Charge-offs:
Gross charge-offs	(63)	(40)	(26)
Expected future recoveries on current period gross charge-offs	0	0	0
Total	(63)	(40)	(26)
Adjustment resulting from the change in charge-off rate	0	0	0
Net charge-offs	(63)	(40)	(26)
Decrease in expected future recoveries on previously fully charged-off loans	0	0	0
Allowance at end of period	$ 215	$ 222	$ 262
Net charge-offs as a percentage of average loans in repayment, excluding the net adjustment resulting from the change in charge-off rate	0.19%	0.10%	0.06%
Net adjustment resulting from the change in charge-off rate as a percentage of average loans in repayment	0.00%	0.00%	0.00%
Net charge-offs as a percentage of average loans in repayment	0.19%	0.10%	0.06%
Ending total loans	$ 38,140	$ 43,747	$ 52,903
Average loans in repayment	33,047	40,332	45,781
Ending loans in repayment	30,436	34,372	44,390
Private Education Loans [Member]
Allowance for Loan Losses
Allowance at beginning of period	800	1,009	1,089
Provision:
Reversal of allowance related to loan sales			(107)
Remaining provision			46
Total provision	67	79	(61)
Charge-offs:
Gross charge-offs	(320)	(370)	(175)
Expected future recoveries on current period gross charge-offs	47	57	22
Total	(273)	(313)	(153)
Adjustment resulting from the change in charge-off rate	(25)	(30)	(16)
Net charge-offs	(298)	(343)	(169)
Decrease in expected future recoveries on previously fully charged-off loans	48	55	150
Allowance at end of period	$ 617	$ 800	$ 1,009
Net charge-offs as a percentage of average loans in repayment, excluding the net adjustment resulting from the change in charge-off rate	1.54%	1.59%	0.76%
Net adjustment resulting from the change in charge-off rate as a percentage of average loans in repayment	0.14%	0.15%	0.08%
Net charge-offs as a percentage of average loans in repayment	1.68%	1.74%	0.84%
Ending total loans	$ 17,519	$ 19,525	$ 21,180
Average loans in repayment	17,749	19,796	20,150
Ending loans in repayment	$ 16,796	$ 18,770	$ 20,284