Allowance for Credit Losses - Age Analysis of Past Due Loans Delinquencies (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans allowance for losses	$ (1,350,058)	$ (1,339,772)	$ (1,479,306)	$ (1,357,075)
Private Education Loans, net	$ 20,200,789	$ 20,306,357
Percentage of Private Education Loans in repayment, Year 1	52.10%	55.60%
Percentage of Private Education Loans in repayment, Year 2	56.30%	65.30%
Percentage of Private Education Loans in repayment, Year 3	66.20%	75.20%
Percentage of Private Education Loans in repayment, Year 4	75.10%	81.10%
Percentage of Private Education Loans in repayment, Year 5	80.60%	85.00%
Percentage of Private Education Loans in repayment, After Year 5	88.00%	89.30%
Percentage of Private Education Loans in repayment	71.40%	73.30%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 1	0.50%	1.00%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 2	1.00%	2.10%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 3	2.80%	3.60%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 4	3.60%	4.10%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 5	3.70%	4.50%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, After Year 5	5.30%	6.80%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment	3.40%	3.90%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 1	0.20%	0.80%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 2	1.00%	2.70%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 3	4.20%	2.60%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 4	3.60%	2.30%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 5	3.20%	2.20%
Loans in forbearance as a percentage of loans in repayment and forbearance, After Year 5	2.20%	1.90%
Loans in forbearance as a percentage of loans in repayment and forbearance	2.50%	2.10%
Consumer Portfolio Segment | Student Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans in Repayment, Year 1	$ 482,998	$ 2,494,814
Total Private Education Loans in Repayment, Year 2	3,267,486	3,323,327
Total Private Education Loans in Repayment, Year 3	2,643,579	2,209,734
Total Private Education Loans in Repayment, Year 4	1,842,892	1,475,858
Total Private Education Loans in Repayment, Year 5	1,294,286	1,386,889
Total Private Education Loans in Repayment, After Year 5	5,430,451	4,519,192
Total Private Education Loans in repayment	14,961,692	15,409,814
Total Private Education Loans, gross, Year 1	926,224	4,490,119
Total Private Education Loans, gross, Year 2	5,803,342	5,088,739
Total Private Education Loans, gross, Year 3	3,994,361	2,937,422
Total Private Education Loans, gross, Year 4	2,454,339	1,819,089
Total Private Education Loans, gross, Year 5	1,606,219	1,632,300
Total Private Education Loans, gross, After Year 5	6,167,861	5,058,175
Total Private Education Loans, gross	20,952,346	21,025,844
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 1	10,902	35,616
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 2	31,667	18,556
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 3	14,460	9,465
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 4	7,834	5,809
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 5	5,078	3,556
Private Education Loans deferred origination costs and unamortized premium/(discount), After Year 5	10,927	8,552
Private Education Loans deferred origination costs and unamortized premium/(discount)	80,868	81,554
Total Private Education Loans, Year 1	937,126	4,525,735
Total Private Education Loans, Year 2	5,835,009	5,107,295
Total Private Education Loans, Year 3	4,008,821	2,946,887
Total Private Education Loans, Year 4	2,462,173	1,824,898
Total Private Education Loans, Year 5	1,611,297	1,635,856
Total Private Education Loans, After Year 5	6,178,788	5,066,727
Total Private Education Loans	21,033,214	21,107,398
Private Education Loans allowance for losses, Year 1	(67,459)	(269,642)
Private Education Loans allowance for losses, Year 2	(354,779)	(335,090)
Private Education Loans allowance for losses, Year 3	(271,004)	(194,104)
Private Education Loans allowance for losses, Year 4	(164,549)	(118,755)
Private Education Loans allowance for losses, Year 5	(105,385)	(100,111)
Private Education Loans allowance for losses, After Year 5	(382,255)	(317,403)
Private Education Loans allowance for losses	(1,345,431)	(1,335,105)
Private Education Loans, net, Year 1	869,667	4,256,093
Private Education Loans, net, Year 2	5,480,230	4,772,205
Private Education Loans, net, Year 3	3,737,817	2,752,783
Private Education Loans, net, Year 4	2,297,624	1,706,143
Private Education Loans, net, Year 5	1,505,912	1,535,745
Private Education Loans, net, After Year 5	5,796,533	4,749,324
Private Education Loans, net	19,687,783	19,772,293
Consumer Portfolio Segment | Student Loan | Loan delinquent, current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	480,422	2,469,817
Year 2	3,236,356	3,254,534
Year 3	2,569,306	2,131,040
Year 4	1,776,854	1,416,069
Year 5	1,245,882	1,323,825
After Year 5	5,142,786	4,213,986
Loan type	14,451,606	14,809,271
Consumer Portfolio Segment | Student Loan | Loan delinquent 30-59 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	2,576	17,599
Year 2	18,739	34,627
Year 3	32,476	37,147
Year 4	30,460	28,020
Year 5	22,553	31,432
After Year 5	133,231	149,926
Loan type	240,035	298,751
Consumer Portfolio Segment | Student Loan | Loan delinquent 60-89 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	0	5,720
Year 2	7,600	17,227
Year 3	20,016	20,077
Year 4	17,424	16,614
Year 5	12,482	15,482
After Year 5	76,399	75,897
Loan type	133,921	151,017
Consumer Portfolio Segment | Student Loan | Loan delinquent 90 days or greater past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	0	1,678
Year 2	4,791	16,939
Year 3	21,781	21,470
Year 4	18,154	15,155
Year 5	13,369	16,150
After Year 5	78,035	79,383
Loan type	136,130	150,775
Loans In-School/Grace/Deferment | Consumer Portfolio Segment | Student Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	442,021	1,976,040
Year 2	2,501,814	1,672,333
Year 3	1,235,053	669,250
Year 4	541,726	307,781
Year 5	268,959	213,593
After Year 5	613,124	452,994
Loan type	5,602,697	5,291,991
Loans In Forbearance | Consumer Portfolio Segment | Student Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans in forbearance, Year 1	1,205	19,265
Total Private Education Loans in forbearance, Year 2	34,042	93,079
Total Private Education Loans in forbearance, Year 3	115,729	58,438
Total Private Education Loans in forbearance, Year 4	69,721	35,450
Total Private Education Loans in forbearance, Year 5	42,974	31,818
Total Private Education Loans in forbearance, After Year 5	124,286	85,989
Total Private Education Loans in forbearance	$ 387,957	$ 324,039