Allowance for Credit Losses and Unfunded Loan Commitments - Age Analysis of Past Due Loans Delinquencies (Details) - USD ($) $ in Thousands	Mar. 31, 2025	Dec. 31, 2024	Mar. 31, 2024	Dec. 31, 2023
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans allowance for losses	$ (1,443,715)	$ (1,435,920)	$ (1,350,058)	$ (1,339,772)
Private Education Loans, net	$ 21,091,204	$ 20,902,158
Percentage of Private Education Loans in repayment, Year 1	53.60%	56.50%
Percentage of Private Education Loans in repayment, Year 2	57.20%	63.50%
Percentage of Private Education Loans in repayment, Year 3	64.20%	73.70%
Percentage of Private Education Loans in repayment, Year 4	73.70%	80.30%
Percentage of Private Education Loans in repayment, Year 5	79.70%	84.30%
Percentage of Private Education Loans in repayment, After Year 5	88.30%	89.70%
Percentage of Private Education Loans in repayment	70.90%	72.40%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 1	0.20%	0.70%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 2	0.60%	2.00%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 3	3.00%	3.80%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 4	4.20%	4.30%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 5	4.50%	4.40%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, After Year 5	5.70%	6.30%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment	3.60%	3.70%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 1	0.20%	0.70%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 2	1.00%	3.60%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 3	5.00%	3.60%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 4	4.50%	3.10%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 5	4.00%	2.70%
Loans in forbearance as a percentage of loans in repayment and forbearance, After Year 5	2.30%	1.80%
Loans in forbearance as a percentage of loans in repayment and forbearance	2.80%	2.50%
Consumer Portfolio Segment | Student Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans in Repayment, Year 1	$ 505,279	$ 2,840,129
Total Private Education Loans in Repayment, Year 2	3,696,844	3,379,077
Total Private Education Loans in Repayment, Year 3	2,659,437	2,348,504
Total Private Education Loans in Repayment, Year 4	2,052,134	1,661,502
Total Private Education Loans in Repayment, Year 5	1,507,821	1,158,044
Total Private Education Loans in Repayment, After Year 5	5,482,282	4,719,495
Total Private Education Loans in repayment	15,903,797	16,106,751
Total Private Education Loans, gross, Year 1	942,378	5,024,592
Total Private Education Loans, gross, Year 2	6,461,366	5,321,510
Total Private Education Loans, gross, Year 3	4,141,150	3,185,247
Total Private Education Loans, gross, Year 4	2,784,645	2,069,436
Total Private Education Loans, gross, Year 5	1,890,758	1,373,996
Total Private Education Loans, gross, After Year 5	6,211,828	5,260,227
Total Private Education Loans, gross	22,432,125	22,235,008
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 1	13,142	47,659
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 2	42,843	25,599
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 3	20,119	10,788
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 4	9,329	6,142
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 5	5,521	4,057
Private Education Loans deferred origination costs and unamortized premium/(discount), After Year 5	11,840	8,825
Private Education Loans deferred origination costs and unamortized premium/(discount)	102,794	103,070
Total Private Education Loans, Year 1	955,520	5,072,251
Total Private Education Loans, Year 2	6,504,209	5,347,109
Total Private Education Loans, Year 3	4,161,269	3,196,035
Total Private Education Loans, Year 4	2,793,974	2,075,578
Total Private Education Loans, Year 5	1,896,279	1,378,053
Total Private Education Loans, After Year 5	6,223,668	5,269,052
Total Private Education Loans	22,534,919	22,338,078
Private Education Loans allowance for losses, Year 1	(36,944)	(258,235)
Private Education Loans allowance for losses, Year 2	(332,859)	(326,207)
Private Education Loans allowance for losses, Year 3	(283,874)	(234,532)
Private Education Loans allowance for losses, Year 4	(217,640)	(150,324)
Private Education Loans allowance for losses, Year 5	(141,990)	(90,600)
Private Education Loans allowance for losses, After Year 5	(430,408)	(376,022)
Private Education Loans allowance for losses	(1,443,715)	(1,435,920)
Private Education Loans, net, Year 1	918,576	4,814,016
Private Education Loans, net, Year 2	6,171,350	5,020,902
Private Education Loans, net, Year 3	3,877,395	2,961,503
Private Education Loans, net, Year 4	2,576,334	1,925,254
Private Education Loans, net, Year 5	1,754,289	1,287,453
Private Education Loans, net, After Year 5	5,793,260	4,893,030
Private Education Loans, net	21,091,204	20,902,158
Consumer Portfolio Segment | Student Loan | Loan delinquent, current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	504,171	2,820,940
Year 2	3,673,471	3,312,916
Year 3	2,578,666	2,259,455
Year 4	1,965,257	1,590,812
Year 5	1,439,732	1,107,189
After Year 5	5,172,375	4,422,021
Loan type	15,333,672	15,513,333
Consumer Portfolio Segment | Student Loan | Loan delinquent 30-59 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	1,108	13,533
Year 2	13,654	36,441
Year 3	35,819	45,543
Year 4	39,736	35,245
Year 5	31,663	27,302
After Year 5	154,299	152,684
Loan type	276,279	310,748
Consumer Portfolio Segment | Student Loan | Loan delinquent 60-89 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	0	3,973
Year 2	5,932	15,239
Year 3	23,863	23,359
Year 4	24,895	18,365
Year 5	19,818	10,921
After Year 5	78,104	68,878
Loan type	152,612	140,735
Consumer Portfolio Segment | Student Loan | Loan delinquent 90 days or greater past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	0	1,683
Year 2	3,787	14,481
Year 3	21,089	20,147
Year 4	22,246	17,080
Year 5	16,608	12,632
After Year 5	77,504	75,912
Loan type	141,234	141,935
Loans In-school/grace/deferment | Consumer Portfolio Segment | Student Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	436,059	2,164,479
Year 2	2,726,541	1,817,705
Year 3	1,341,863	748,782
Year 4	635,952	355,248
Year 5	320,015	184,377
After Year 5	603,297	452,236
Loan type	6,063,727	5,722,827
Loans In forbearance | Consumer Portfolio Segment | Student Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans in forbearance, Year 1	1,040	19,984
Total Private Education Loans in forbearance, Year 2	37,981	124,728
Total Private Education Loans in forbearance, Year 3	139,850	87,961
Total Private Education Loans in forbearance, Year 4	96,559	52,686
Total Private Education Loans in forbearance, Year 5	62,922	31,575
Total Private Education Loans in forbearance, After Year 5	126,249	88,496
Total Private Education Loans in forbearance	$ 464,601	$ 405,430