Allowance for Credit Losses - Age Analysis of Past Due Loans Delinquencies (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans allowance for losses	$ (1,339,772)	$ (1,357,075)	$ (1,165,335)	$ (1,361,723)
Private Education Loans, net	$ 20,306,357	$ 19,626,868
Percentage of Private Education Loans in repayment, Year 1	55.60%	56.90%	59.00%
Percentage of Private Education Loans in repayment, Year 2	65.30%	67.10%	67.90%
Percentage of Private Education Loans in repayment, Year 3	75.20%	76.10%	74.30%
Percentage of Private Education Loans in repayment, Year 4	81.10%	80.70%	79.30%
Percentage of Private Education Loans in repayment, Year 5	85.00%	84.50%	82.90%
Percentage of Private Education Loans in repayment, Year 6	89.30%	89.30%	87.40%
Percentage of Private Education Loans in repayment	73.30%	74.50%	74.90%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 1	1.00%	0.90%	0.80%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 2	2.10%	1.90%	1.40%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 3	3.60%	3.10%	2.90%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 4	4.10%	4.00%	3.70%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 5	4.50%	4.70%	4.30%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 6	6.80%	6.50%	5.20%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, total	3.90%	3.80%	3.30%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 1	0.80%	0.70%	0.50%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 2	2.70%	2.10%	1.90%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 3	2.60%	1.90%	2.10%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 4	2.30%	2.10%	2.30%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 5	2.20%	2.20%	2.50%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 6	1.90%	2.00%	2.10%
Loans in forbearance as a percentage of loans in repayment and forbearance	2.10%	1.80%	1.90%
Student Loan | Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans in forbearance, Year 1	$ 2,494,814	$ 2,432,838	$ 2,253,860
Total Private Education Loans in forbearance, Year 2	3,323,327	3,049,477	2,826,916
Total Private Education Loans in forbearance, Year 3	2,209,734	1,967,756	2,390,841
Total Private Education Loans in forbearance, Year 4	1,475,858	1,754,219	1,840,790
Total Private Education Loans in forbearance, Year 5	1,386,889	1,365,979	1,640,870
Total Private Education Loans in forbearance, Year 6	4,519,192	4,559,281	4,557,935
Private Education Loans in forbearance, total	15,409,814	15,129,550	15,511,212
Total Private Education Loans, gross, Year 1	4,490,119	4,276,533	3,822,361
Total Private Education Loans, gross, Year 2	5,088,739	4,547,159	4,166,283
Total Private Education Loans, gross, Year 3	2,937,422	2,584,622	3,216,525
Total Private Education Loans, gross, Year 4	1,819,089	2,172,660	2,320,060
Total Private Education Loans, gross, Year 5	1,632,300	1,615,842	1,978,233
Total Private Education Loans, gross, Year 6	5,058,175	5,106,872	5,213,401
Total Private Education Loans, gross	21,025,844	20,303,688	20,716,863
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 1	35,616	26,714	22,169
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 2	18,556	15,933	16,067
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 3	9,465	9,062	9,575
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 4	5,809	5,496	5,918
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 5	3,556	3,575	4,588
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 6	8,552	8,876	9,171
Private Education Loans deferred origination costs and unamortized premium/(discount), Total	81,554	69,656	67,488
Total Private Education Loans, Year 1	4,525,735	4,303,247	3,844,530
Total Private Education Loans, Year 2	5,107,295	4,563,092	4,182,350
Total Private Education Loans, Year 3	2,946,887	2,593,684	3,226,100
Total Private Education Loans, Year 4	1,824,898	2,178,156	2,325,978
Total Private Education Loans, Year 5	1,635,856	1,619,417	1,982,821
Total Private Education Loans, Year 6	5,066,727	5,115,748	5,222,572
Total Private Education Loans	21,107,398	20,373,344	20,784,351
Private Education Loans allowance for losses, Year 1	(269,642)	(304,943)	(248,102)
Private Education Loans allowance for losses, Year 2	(335,090)	(323,506)	(239,507)
Private Education Loans allowance for losses, Year 3	(194,104)	(181,915)	(195,223)
Private Education Loans allowance for losses, Year 4	(118,755)	(141,424)	(129,678)
Private Education Loans allowance for losses, Year 5	(100,111)	(101,023)	(99,982)
Private Education Loans allowance for losses, Year 6	(317,403)	(300,820)	(246,485)
Private Education Loans allowance for losses	(1,335,105)	(1,353,631)	(1,158,977)
Private Education Loans, net, Year 1	4,256,093	3,998,304	3,596,428
Private Education Loans, net, Year 2	4,772,205	4,239,586	3,942,843
Private Education Loans, net, Year 3	2,752,783	2,411,769	3,030,877
Private Education Loans, net, Year 4	1,706,143	2,036,732	2,196,300
Private Education Loans, net, Year 5	1,535,745	1,518,394	1,882,839
Private Education Loans, net, Year 6	4,749,324	4,814,928	4,976,087
Private Education Loans, net	19,772,293	19,019,713	19,625,374
Student Loan | Consumer Portfolio Segment | Loan delinquent, current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	2,469,817	2,411,441	2,234,876
Year 2	3,254,534	2,991,839	2,786,646
Year 3	2,131,040	1,907,574	2,321,728
Year 4	1,416,069	1,683,986	1,772,651
Year 5	1,323,825	1,301,809	1,570,815
After Year 5	4,213,986	4,262,698	4,319,057
Loan type	14,809,271	14,559,347	15,005,773
Student Loan | Consumer Portfolio Segment | Loan delinquent 30-59 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	17,599	14,164	15,148
Year 2	34,627	30,740	29,146
Year 3	37,147	30,877	46,616
Year 4	28,020	35,213	43,197
Year 5	31,432	31,366	41,695
After Year 5	149,926	144,948	132,757
Loan type	298,751	287,308	308,559
Student Loan | Consumer Portfolio Segment | Loan delinquent 60-89 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	5,720	5,523	3,194
Year 2	17,227	15,056	7,441
Year 3	20,077	14,433	14,044
Year 4	16,614	18,201	14,310
Year 5	15,482	16,697	16,425
After Year 5	75,897	77,595	61,533
Loan type	151,017	147,505	116,947
Student Loan | Consumer Portfolio Segment | Loan delinquent 90 days or greater past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	1,678	1,710	642
Year 2	16,939	11,842	3,683
Year 3	21,470	14,872	8,453
Year 4	15,155	16,819	10,632
Year 5	16,150	16,107	11,935
After Year 5	79,383	74,040	44,588
Loan type	150,775	135,390	79,933
Student Loan | Loans In-School/Grace/Deferment | Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	1,976,040	1,827,649	1,556,550
Year 2	1,672,333	1,433,322	1,283,523
Year 3	669,250	578,253	773,320
Year 4	307,781	380,639	435,657
Year 5	213,593	219,280	296,008
After Year 5	452,994	455,910	559,356
Loan type	5,291,991	4,895,053	4,904,414
Student Loan | Loans In Forbearance | Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans in forbearance, Year 1	19,265	16,046	11,951
Total Private Education Loans in forbearance, Year 2	93,079	64,360	55,844
Total Private Education Loans in forbearance, Year 3	58,438	38,613	52,364
Total Private Education Loans in forbearance, Year 4	35,450	37,802	43,613
Total Private Education Loans in forbearance, Year 5	31,818	30,583	41,355
Total Private Education Loans in forbearance, Year 6	85,989	91,681	96,110
Private Education Loans in forbearance, total	$ 324,039	$ 279,085	$ 301,237