Allowance for Credit Losses and Unfunded Loan Commitments - Age Analysis of Past Due Loans Delinquencies (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans allowance for losses	$ (1,435,920)	$ (1,339,772)	$ (1,357,075)	$ (1,165,335)
Private Education Loans, net	$ 20,902,158	$ 20,306,357
Percentage of Private Education Loans in repayment, Year 1	56.50%	55.60%	56.90%
Percentage of Private Education Loans in repayment, Year 2	63.50%	65.30%	67.10%
Percentage of Private Education Loans in repayment, Year 3	73.70%	75.20%	76.10%
Percentage of Private Education Loans in repayment, Year 4	80.30%	81.10%	80.70%
Percentage of Private Education Loans in repayment, Year 5	84.30%	85.00%	84.50%
Percentage of Private Education Loans in repayment, Year 6	89.70%	89.30%	89.30%
Percentage of Private Education Loans in repayment	72.40%	73.30%	74.50%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 1	0.70%	1.00%	0.90%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 2	2.00%	2.10%	1.90%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 3	3.80%	3.60%	3.10%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 4	4.30%	4.10%	4.00%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 5	4.40%	4.50%	4.70%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 6	6.30%	6.80%	6.50%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, total	3.70%	3.90%	3.80%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 1	0.70%	0.80%	0.70%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 2	3.60%	2.70%	2.10%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 3	3.60%	2.60%	1.90%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 4	3.10%	2.30%	2.10%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 5	2.70%	2.20%	2.20%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 6	1.80%	1.90%	2.00%
Loans in forbearance as a percentage of loans in repayment and forbearance	2.50%	2.10%	1.80%
Student Loan | Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans in forbearance, Year 1	$ 2,840,129	$ 2,494,814	$ 2,432,838
Total Private Education Loans in forbearance, Year 2	3,379,077	3,323,327	3,049,477
Total Private Education Loans in forbearance, Year 3	2,348,504	2,209,734	1,967,756
Total Private Education Loans in forbearance, Year 4	1,661,502	1,475,858	1,754,219
Total Private Education Loans in forbearance, Year 5	1,158,044	1,386,889	1,365,979
Total Private Education Loans in forbearance, Year 6	4,719,495	4,519,192	4,559,281
Private Education Loans in forbearance, total	16,106,751	15,409,814	15,129,550
Total Private Education Loans, gross, Year 1	5,024,592	4,490,119	4,276,533
Total Private Education Loans, gross, Year 2	5,321,510	5,088,739	4,547,159
Total Private Education Loans, gross, Year 3	3,185,247	2,937,422	2,584,622
Total Private Education Loans, gross, Year 4	2,069,436	1,819,089	2,172,660
Total Private Education Loans, gross, Year 5	1,373,996	1,632,300	1,615,842
Total Private Education Loans, gross, Year 6	5,260,227	5,058,175	5,106,872
Total Private Education Loans, gross	22,235,008	21,025,844	20,303,688
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 1	47,659	35,616	26,714
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 2	25,599	18,556	15,933
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 3	10,788	9,465	9,062
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 4	6,142	5,809	5,496
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 5	4,057	3,556	3,575
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 6	8,825	8,552	8,876
Private Education Loans deferred origination costs and unamortized premium/(discount), Total	103,070	81,554	69,656
Total Private Education Loans, Year 1	5,072,251	4,525,735	4,303,247
Total Private Education Loans, Year 2	5,347,109	5,107,295	4,563,092
Total Private Education Loans, Year 3	3,196,035	2,946,887	2,593,684
Total Private Education Loans, Year 4	2,075,578	1,824,898	2,178,156
Total Private Education Loans, Year 5	1,378,053	1,635,856	1,619,417
Total Private Education Loans, Year 6	5,269,052	5,066,727	5,115,748
Total Private Education Loans	22,338,078	21,107,398	20,373,344
Private Education Loans allowance for losses, Year 1	(258,235)	(269,642)	(304,943)
Private Education Loans allowance for losses, Year 2	(326,207)	(335,090)	(323,506)
Private Education Loans allowance for losses, Year 3	(234,532)	(194,104)	(181,915)
Private Education Loans allowance for losses, Year 4	(150,324)	(118,755)	(141,424)
Private Education Loans allowance for losses, Year 5	(90,600)	(100,111)	(101,023)
Private Education Loans allowance for losses, Year 6	(376,022)	(317,403)	(300,820)
Private Education Loans allowance for losses	(1,435,920)	(1,335,105)	(1,353,631)
Private Education Loans, net, Year 1	4,814,016	4,256,093	3,998,304
Private Education Loans, net, Year 2	5,020,902	4,772,205	4,239,586
Private Education Loans, net, Year 3	2,961,503	2,752,783	2,411,769
Private Education Loans, net, Year 4	1,925,254	1,706,143	2,036,732
Private Education Loans, net, Year 5	1,287,453	1,535,745	1,518,394
Private Education Loans, net, Year 6	4,893,030	4,749,324	4,814,928
Private Education Loans, net	20,902,158	19,772,293	19,019,713
Student Loan | Consumer Portfolio Segment | Loan delinquent, current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	2,820,940	2,469,817	2,411,441
Year 2	3,312,916	3,254,534	2,991,839
Year 3	2,259,455	2,131,040	1,907,574
Year 4	1,590,812	1,416,069	1,683,986
Year 5	1,107,189	1,323,825	1,301,809
After Year 5	4,422,021	4,213,986	4,262,698
Loan type	15,513,333	14,809,271	14,559,347
Student Loan | Consumer Portfolio Segment | Loan delinquent 30-59 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	13,533	17,599	14,164
Year 2	36,441	34,627	30,740
Year 3	45,543	37,147	30,877
Year 4	35,245	28,020	35,213
Year 5	27,302	31,432	31,366
After Year 5	152,684	149,926	144,948
Loan type	310,748	298,751	287,308
Student Loan | Consumer Portfolio Segment | Loan delinquent 60-89 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	3,973	5,720	5,523
Year 2	15,239	17,227	15,056
Year 3	23,359	20,077	14,433
Year 4	18,365	16,614	18,201
Year 5	10,921	15,482	16,697
After Year 5	68,878	75,897	77,595
Loan type	140,735	151,017	147,505
Student Loan | Consumer Portfolio Segment | Loan delinquent 90 days or greater past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	1,683	1,678	1,710
Year 2	14,481	16,939	11,842
Year 3	20,147	21,470	14,872
Year 4	17,080	15,155	16,819
Year 5	12,632	16,150	16,107
After Year 5	75,912	79,383	74,040
Loan type	141,935	150,775	135,390
Student Loan | Loans In-School/Grace/Deferment | Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	2,164,479	1,976,040	1,827,649
Year 2	1,817,705	1,672,333	1,433,322
Year 3	748,782	669,250	578,253
Year 4	355,248	307,781	380,639
Year 5	184,377	213,593	219,280
After Year 5	452,236	452,994	455,910
Loan type	5,722,827	5,291,991	4,895,053
Student Loan | Loans In Forbearance | Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans in forbearance, Year 1	19,984	19,265	16,046
Total Private Education Loans in forbearance, Year 2	124,728	93,079	64,360
Total Private Education Loans in forbearance, Year 3	87,961	58,438	38,613
Total Private Education Loans in forbearance, Year 4	52,686	35,450	37,802
Total Private Education Loans in forbearance, Year 5	31,575	31,818	30,583
Total Private Education Loans in forbearance, Year 6	88,496	85,989	91,681
Private Education Loans in forbearance, total	$ 405,430	$ 324,039	$ 279,085