Allowance for Loan Losses - Private Education Loan Portfolio Stratified by Key Credit Quality Indicators (Detail) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Accounts Notes And Loans Receivable [Line Items]
Loan allowance for losses	$ (832)	$ (1,022)	$ (1,271)	$ (1,377)
Loans, net	54,827	62,250
Private Education Loans [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Loans, gross	17,519	19,525	21,180
Loan allowance for losses	(617)	(800)	$ (1,009)	$ (1,089)
Loans, net	16,902	18,725
Loan, Net Charge-Offs in Year One	0
Loan, Net Charge-Offs in Year Two	(7)
Loan, Net Charge-Offs in Year Three	(10)
Loan, Net Charge-Offs in Year Four	(5)
Loan, Net Charge-Offs in Year Five	(7)
Loan, Net Charge-Offs in Year Prior	(269)
Total Charge-Offs	$ (298)
Percentage of Loan, Net Charge-Offs	100.00%
FICO Scores [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Private Education Loan Credit Quality Indicators by Origination Year One	$ 826	1,756
Total Private Education Loan Credit Quality Indicators by Origination Year Two	1,642	4,614
Total Private Education Loan Credit Quality Indicators by Origination Year Three	4,020	1,538
Total Private Education Loan Credit Quality Indicators by Origination Year Four	1,269	1,467
Total Private Education Loan Credit Quality Indicators by Origination Year Five	1,211	548
Total Private Education Loan Credit Quality Indicators by Origination Year Prior	8,551	9,602
Total Loans, gross	$ 17,519	$ 19,525
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent	100.00%	100.00%
FICO Scores [Member] | 640 and Above [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Private Education Loan Credit Quality Indicators by Origination Year One	$ 815	$ 1,721
Total Private Education Loan Credit Quality Indicators by Origination Year Two	1,575	4,529
Total Private Education Loan Credit Quality Indicators by Origination Year Three	3,898	1,515
Total Private Education Loan Credit Quality Indicators by Origination Year Four	1,240	1,426
Total Private Education Loan Credit Quality Indicators by Origination Year Five	1,162	529
Total Private Education Loan Credit Quality Indicators by Origination Year Prior	7,132	8,107
Total Loans, gross	$ 15,822	$ 17,827
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent	90.00%	91.00%
FICO Scores [Member] | Below 640 [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Private Education Loan Credit Quality Indicators by Origination Year One	$ 11	$ 35
Total Private Education Loan Credit Quality Indicators by Origination Year Two	67	85
Total Private Education Loan Credit Quality Indicators by Origination Year Three	122	23
Total Private Education Loan Credit Quality Indicators by Origination Year Four	29	41
Total Private Education Loan Credit Quality Indicators by Origination Year Five	49	19
Total Private Education Loan Credit Quality Indicators by Origination Year Prior	1,419	1,495
Total Loans, gross	$ 1,697	$ 1,698
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent	10.00%	9.00%
Loan Status [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Private Education Loan Credit Quality Indicators by Origination Year One	$ 826	$ 1,756
Total Private Education Loan Credit Quality Indicators by Origination Year Two	1,642	4,614
Total Private Education Loan Credit Quality Indicators by Origination Year Three	4,020	1,538
Total Private Education Loan Credit Quality Indicators by Origination Year Four	1,269	1,467
Total Private Education Loan Credit Quality Indicators by Origination Year Five	1,211	548
Total Private Education Loan Credit Quality Indicators by Origination Year Prior	8,551	9,602
Total Loans, gross	$ 17,519	$ 19,525
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent	100.00%	100.00%
Loan Status [Member] | School/Grace/Deferment [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Private Education Loan Credit Quality Indicators by Origination Year One	$ 51	$ 56
Total Private Education Loan Credit Quality Indicators by Origination Year Two	79	91
Total Private Education Loan Credit Quality Indicators by Origination Year Three	87	29
Total Private Education Loan Credit Quality Indicators by Origination Year Four	21	33
Total Private Education Loan Credit Quality Indicators by Origination Year Five	27	13
Total Private Education Loan Credit Quality Indicators by Origination Year Prior	458	533
Total Loans, gross	$ 723	$ 755
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent	4.00%	4.00%
Loan Status [Member] | Current/90 Days or Less Delinquent [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Private Education Loan Credit Quality Indicators by Origination Year One	$ 774	$ 1,697
Total Private Education Loan Credit Quality Indicators by Origination Year Two	1,554	4,514
Total Private Education Loan Credit Quality Indicators by Origination Year Three	3,917	1,506
Total Private Education Loan Credit Quality Indicators by Origination Year Four	1,244	1,428
Total Private Education Loan Credit Quality Indicators by Origination Year Five	1,177	533
Total Private Education Loan Credit Quality Indicators by Origination Year Prior	7,750	8,681
Total Loans, gross	$ 16,416	$ 18,359
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent	94.00%	94.00%
Loan Status [Member] | Greater than 90 days Delinquent [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Private Education Loan Credit Quality Indicators by Origination Year One	$ 1	$ 3
Total Private Education Loan Credit Quality Indicators by Origination Year Two	9	9
Total Private Education Loan Credit Quality Indicators by Origination Year Three	16	3
Total Private Education Loan Credit Quality Indicators by Origination Year Four	4	6
Total Private Education Loan Credit Quality Indicators by Origination Year Five	7	2
Total Private Education Loan Credit Quality Indicators by Origination Year Prior	343	388
Total Loans, gross	$ 380	$ 411
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent	2.00%	2.00%
Seasoning [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Private Education Loan Credit Quality Indicators by Origination Year One	$ 826	$ 1,756
Total Private Education Loan Credit Quality Indicators by Origination Year Two	1,642	4,614
Total Private Education Loan Credit Quality Indicators by Origination Year Three	4,020	1,538
Total Private Education Loan Credit Quality Indicators by Origination Year Four	1,269	1,467
Total Private Education Loan Credit Quality Indicators by Origination Year Five	1,211	548
Total Private Education Loan Credit Quality Indicators by Origination Year Prior	8,551	9,602
Total Loans, gross	$ 17,519	$ 19,525
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent	100.00%	100.00%
Seasoning [Member] | 1-12 Payments [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Private Education Loan Credit Quality Indicators by Origination Year One	$ 781	$ 1,714
Total Private Education Loan Credit Quality Indicators by Origination Year Two	130	860
Total Private Education Loan Credit Quality Indicators by Origination Year Three	28	10
Total Private Education Loan Credit Quality Indicators by Origination Year Four	7	9
Total Private Education Loan Credit Quality Indicators by Origination Year Five	4	1
Total Private Education Loan Credit Quality Indicators by Origination Year Prior	54	83
Total Loans, gross	$ 1,004	$ 2,677
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent	6.00%	14.00%
Seasoning [Member] | 13-24 Payments [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Private Education Loan Credit Quality Indicators by Origination Year One	$ 0	$ 0
Total Private Education Loan Credit Quality Indicators by Origination Year Two	1,453	3,699
Total Private Education Loan Credit Quality Indicators by Origination Year Three	812	147
Total Private Education Loan Credit Quality Indicators by Origination Year Four	12	31
Total Private Education Loan Credit Quality Indicators by Origination Year Five	13	6
Total Private Education Loan Credit Quality Indicators by Origination Year Prior	62	91
Total Loans, gross	$ 2,352	$ 3,974
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent	13.00%	20.00%
Seasoning [Member] | 25-36 Payments [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Private Education Loan Credit Quality Indicators by Origination Year One	$ 0	$ 0
Total Private Education Loan Credit Quality Indicators by Origination Year Two	0	0
Total Private Education Loan Credit Quality Indicators by Origination Year Three	3,127	1,365
Total Private Education Loan Credit Quality Indicators by Origination Year Four	142	452
Total Private Education Loan Credit Quality Indicators by Origination Year Five	35	15
Total Private Education Loan Credit Quality Indicators by Origination Year Prior	113	161
Total Loans, gross	$ 3,417	$ 1,993
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent	20.00%	10.00%
Seasoning [Member] | 37-48 Payments [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Private Education Loan Credit Quality Indicators by Origination Year One	$ 0	$ 0
Total Private Education Loan Credit Quality Indicators by Origination Year Two	0	0
Total Private Education Loan Credit Quality Indicators by Origination Year Three	0	0
Total Private Education Loan Credit Quality Indicators by Origination Year Four	1,097	957
Total Private Education Loan Credit Quality Indicators by Origination Year Five	388	132
Total Private Education Loan Credit Quality Indicators by Origination Year Prior	179	260
Total Loans, gross	$ 1,664	$ 1,349
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent	9.00%	7.00%
Seasoning [Member] | More than 48 Payments [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Private Education Loan Credit Quality Indicators by Origination Year One	$ 0	$ 0
Total Private Education Loan Credit Quality Indicators by Origination Year Two	0	0
Total Private Education Loan Credit Quality Indicators by Origination Year Three	0	0
Total Private Education Loan Credit Quality Indicators by Origination Year Four	0	0
Total Private Education Loan Credit Quality Indicators by Origination Year Five	759	388
Total Private Education Loan Credit Quality Indicators by Origination Year Prior	7,963	8,790
Total Loans, gross	$ 8,722	$ 9,178
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent	50.00%	47.00%
Seasoning [Member] | School/Grace/Deferment [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Private Education Loan Credit Quality Indicators by Origination Year One	$ 45	$ 42
Total Private Education Loan Credit Quality Indicators by Origination Year Two	59	55
Total Private Education Loan Credit Quality Indicators by Origination Year Three	53	16
Total Private Education Loan Credit Quality Indicators by Origination Year Four	11	18
Total Private Education Loan Credit Quality Indicators by Origination Year Five	12	6
Total Private Education Loan Credit Quality Indicators by Origination Year Prior	180	217
Total Loans, gross	$ 360	$ 354
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent	2.00%	2.00%
Certain Loan Modifications [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Private Education Loan Credit Quality Indicators by Origination Year One	$ 826	$ 1,756
Total Private Education Loan Credit Quality Indicators by Origination Year Two	1,642	4,614
Total Private Education Loan Credit Quality Indicators by Origination Year Three	4,020	1,538
Total Private Education Loan Credit Quality Indicators by Origination Year Four	1,269	1,467
Total Private Education Loan Credit Quality Indicators by Origination Year Five	1,211	548
Total Private Education Loan Credit Quality Indicators by Origination Year Prior	8,551	9,602
Total Loans, gross	$ 17,519	$ 19,525
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent	100.00%	100.00%
Certain Loan Modifications [Member] | Modified Loans [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Private Education Loan Credit Quality Indicators by Origination Year One	$ 0	$ 11
Total Private Education Loan Credit Quality Indicators by Origination Year Two	41	71
Total Private Education Loan Credit Quality Indicators by Origination Year Three	130	34
Total Private Education Loan Credit Quality Indicators by Origination Year Four	46	68
Total Private Education Loan Credit Quality Indicators by Origination Year Five	82	37
Total Private Education Loan Credit Quality Indicators by Origination Year Prior	5,775	6,447
Total Loans, gross	$ 6,074	$ 6,668
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent	35.00%	34.00%
Certain Loan Modifications [Member] | Non-modified Loans [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Private Education Loan Credit Quality Indicators by Origination Year One	$ 826	$ 1,745
Total Private Education Loan Credit Quality Indicators by Origination Year Two	1,601	4,543
Total Private Education Loan Credit Quality Indicators by Origination Year Three	3,890	1,504
Total Private Education Loan Credit Quality Indicators by Origination Year Four	1,223	1,399
Total Private Education Loan Credit Quality Indicators by Origination Year Five	1,129	511
Total Private Education Loan Credit Quality Indicators by Origination Year Prior	2,776	3,155
Total Loans, gross	$ 11,445	$ 12,857
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent	65.00%	66.00%
Cosigners [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Private Education Loan Credit Quality Indicators by Origination Year One	$ 826	$ 1,756
Total Private Education Loan Credit Quality Indicators by Origination Year Two	1,642	4,614
Total Private Education Loan Credit Quality Indicators by Origination Year Three	4,020	1,538
Total Private Education Loan Credit Quality Indicators by Origination Year Four	1,269	1,467
Total Private Education Loan Credit Quality Indicators by Origination Year Five	1,211	548
Total Private Education Loan Credit Quality Indicators by Origination Year Prior	8,551	9,602
Total Loans, gross	$ 17,519	$ 19,525
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent	100.00%	100.00%
Cosigners [Member] | With Cosigner [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Private Education Loan Credit Quality Indicators by Origination Year One	$ 202	$ 122
Total Private Education Loan Credit Quality Indicators by Origination Year Two	179	106
Total Private Education Loan Credit Quality Indicators by Origination Year Three	93	27
Total Private Education Loan Credit Quality Indicators by Origination Year Four	23	10
Total Private Education Loan Credit Quality Indicators by Origination Year Five	9	0
Total Private Education Loan Credit Quality Indicators by Origination Year Prior	5,206	6,184
Total Loans, gross	$ 5,712	$ 6,449
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent	33.00%	33.00%
Cosigners [Member] | Without Cosigner [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Private Education Loan Credit Quality Indicators by Origination Year One	$ 624	$ 1,634
Total Private Education Loan Credit Quality Indicators by Origination Year Two	1,463	4,508
Total Private Education Loan Credit Quality Indicators by Origination Year Three	3,927	1,511
Total Private Education Loan Credit Quality Indicators by Origination Year Four	1,246	1,457
Total Private Education Loan Credit Quality Indicators by Origination Year Five	1,202	548
Total Private Education Loan Credit Quality Indicators by Origination Year Prior	3,345	3,418
Total Loans, gross	$ 11,807	$ 13,076
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent	67.00%	67.00%
School Type [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Private Education Loan Credit Quality Indicators by Origination Year One	$ 826	$ 1,756
Total Private Education Loan Credit Quality Indicators by Origination Year Two	1,642	4,614
Total Private Education Loan Credit Quality Indicators by Origination Year Three	4,020	1,538
Total Private Education Loan Credit Quality Indicators by Origination Year Four	1,269	1,467
Total Private Education Loan Credit Quality Indicators by Origination Year Five	1,211	548
Total Private Education Loan Credit Quality Indicators by Origination Year Prior	8,551	9,602
Total Loans, gross	$ 17,519	$ 19,525
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent	100.00%	100.00%
School Type [Member] | Not For Profit [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Private Education Loan Credit Quality Indicators by Origination Year One	$ 777	$ 1,655
Total Private Education Loan Credit Quality Indicators by Origination Year Two	1,555	4,347
Total Private Education Loan Credit Quality Indicators by Origination Year Three	3,786	1,470
Total Private Education Loan Credit Quality Indicators by Origination Year Four	1,213	1,366
Total Private Education Loan Credit Quality Indicators by Origination Year Five	1,126	503
Total Private Education Loan Credit Quality Indicators by Origination Year Prior	7,225	8,026
Total Loans, gross	$ 15,682	$ 17,367
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent	90.00%	89.00%
School Type [Member] | For Profit [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Private Education Loan Credit Quality Indicators by Origination Year One	$ 49	$ 101
Total Private Education Loan Credit Quality Indicators by Origination Year Two	87	267
Total Private Education Loan Credit Quality Indicators by Origination Year Three	234	68
Total Private Education Loan Credit Quality Indicators by Origination Year Four	56	101
Total Private Education Loan Credit Quality Indicators by Origination Year Five	85	45
Total Private Education Loan Credit Quality Indicators by Origination Year Prior	1,326	1,576
Total Loans, gross	$ 1,837	$ 2,158
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent	10.00%	11.00%