Allowance for Credit Losses - Age Analysis of Past Due Loans Delinquencies (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans allowance for losses	$ (1,269,652)	$ (1,350,058)	$ (1,339,772)	$ (1,364,716)	$ (1,479,306)	$ (1,357,075)
Private Education Loans, net	$ 18,915,333		$ 20,306,357
Percentage of Private Education Loans in repayment, Year 1	52.00%		55.60%
Percentage of Private Education Loans in repayment, Year 2	57.80%		65.30%
Percentage of Private Education Loans in repayment, Year 3	69.20%		75.20%
Percentage of Private Education Loans in repayment, Year 4	77.60%		81.10%
Percentage of Private Education Loans in repayment, Year 5	82.60%		85.00%
Percentage of Private Education Loans in repayment, After Year 5	89.10%		89.30%
Percentage of Private Education Loans in repayment	72.50%		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	1.10%		2.10%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 3	2.90%		3.60%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 4	3.50%		4.10%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 5	3.50%		4.50%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, After Year 5	5.20%		6.80%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment	3.30%		3.90%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 1	0.30%		0.80%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 2	1.10%		2.70%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 3	2.60%		2.60%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 4	2.10%		2.30%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 5	2.00%		2.20%
Loans in forbearance as a percentage of loans in repayment and forbearance, After Year 5	1.90%		1.90%
Loans in forbearance as a percentage of loans in repayment and forbearance	1.80%		2.10%
Consumer Portfolio Segment | Student Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans in Repayment, Year 1	$ 769,857		$ 2,494,814
Total Private Education Loans in Repayment, Year 2	3,134,870		3,323,327
Total Private Education Loans in Repayment, Year 3	2,281,790		2,209,734
Total Private Education Loans in Repayment, Year 4	1,688,961		1,475,858
Total Private Education Loans in Repayment, Year 5	1,215,583		1,386,889
Total Private Education Loans in Repayment, After Year 5	5,140,520		4,519,192
Total Private Education Loans in repayment	14,231,581		15,409,814
Total Private Education Loans, gross, Year 1	1,480,348		4,490,119
Total Private Education Loans, gross, Year 2	5,423,019		5,088,739
Total Private Education Loans, gross, Year 3	3,297,769		2,937,422
Total Private Education Loans, gross, Year 4	2,177,720		1,819,089
Total Private Education Loans, gross, Year 5	1,472,239		1,632,300
Total Private Education Loans, gross, After Year 5	5,768,436		5,058,175
Total Private Education Loans, gross	19,619,531		21,025,844
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 1	17,774		35,616
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 2	27,568		18,556
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 3	11,897		9,465
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 4	6,845		5,809
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 5	4,575		3,556
Private Education Loans deferred origination costs and unamortized premium/(discount), After Year 5	10,002		8,552
Private Education Loans deferred origination costs and unamortized premium/(discount)	78,661		81,554
Total Private Education Loans, Year 1	1,498,122		4,525,735
Total Private Education Loans, Year 2	5,450,587		5,107,295
Total Private Education Loans, Year 3	3,309,666		2,946,887
Total Private Education Loans, Year 4	2,184,565		1,824,898
Total Private Education Loans, Year 5	1,476,814		1,635,856
Total Private Education Loans, After Year 5	5,778,438		5,066,727
Total Private Education Loans	19,698,192		21,107,398
Private Education Loans allowance for losses, Year 1	(80,912)		(269,642)
Private Education Loans allowance for losses, Year 2	(304,066)		(335,090)
Private Education Loans allowance for losses, Year 3	(228,421)		(194,104)
Private Education Loans allowance for losses, Year 4	(152,123)		(118,755)
Private Education Loans allowance for losses, Year 5	(94,330)		(100,111)
Private Education Loans allowance for losses, After Year 5	(405,740)		(317,403)
Private Education Loans allowance for losses	(1,265,592)		(1,335,105)
Private Education Loans, net, Year 1	1,417,210		4,256,093
Private Education Loans, net, Year 2	5,146,521		4,772,205
Private Education Loans, net, Year 3	3,081,245		2,752,783
Private Education Loans, net, Year 4	2,032,442		1,706,143
Private Education Loans, net, Year 5	1,382,484		1,535,745
Private Education Loans, net, After Year 5	5,372,698		4,749,324
Private Education Loans, net	18,432,600		19,772,293
Consumer Portfolio Segment | Student Loan | Loan delinquent, current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	765,729		2,469,817
Year 2	3,100,292		3,254,534
Year 3	2,215,133		2,131,040
Year 4	1,630,195		1,416,069
Year 5	1,172,591		1,323,825
After Year 5	4,872,598		4,213,986
Loan type	13,756,538		14,809,271
Consumer Portfolio Segment | Student Loan | Loan delinquent 30-59 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	2,542		17,599
Year 2	18,637		34,627
Year 3	29,618		37,147
Year 4	27,766		28,020
Year 5	19,455		31,432
After Year 5	126,427		149,926
Loan type	224,445		298,751
Consumer Portfolio Segment | Student Loan | Loan delinquent 60-89 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	759		5,720
Year 2	9,069		17,227
Year 3	19,011		20,077
Year 4	15,343		16,614
Year 5	11,809		15,482
After Year 5	69,393		75,897
Loan type	125,384		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	827		1,678
Year 2	6,872		16,939
Year 3	18,028		21,470
Year 4	15,657		15,155
Year 5	11,728		16,150
After Year 5	72,102		79,383
Loan type	125,214		150,775
Loans In-School/Grace/Deferment | Consumer Portfolio Segment | Student Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	708,196		1,976,040
Year 2	2,254,427		1,672,333
Year 3	955,723		669,250
Year 4	452,970		307,781
Year 5	231,555		213,593
After Year 5	525,887		452,994
Loan type	5,128,758		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	2,295		19,265
Total Private Education Loans in forbearance, Year 2	33,722		93,079
Total Private Education Loans in forbearance, Year 3	60,256		58,438
Total Private Education Loans in forbearance, Year 4	35,789		35,450
Total Private Education Loans in forbearance, Year 5	25,101		31,818
Total Private Education Loans in forbearance, After Year 5	102,029		85,989
Total Private Education Loans in forbearance	$ 259,192		$ 324,039