Allowance for Credit Losses - Age Analysis of Past Due Loans Delinquencies (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans allowance for losses	$ (1,413,621)	$ (1,269,652)	$ (1,339,772)	$ (1,416,048)	$ (1,364,716)	$ (1,357,075)
Private Education Loans, net	$ 20,459,933		$ 20,306,357
Percentage of Private Education Loans in repayment, Year 1	54.80%		55.60%
Percentage of Private Education Loans in repayment, Year 2	58.50%		65.30%
Percentage of Private Education Loans in repayment, Year 3	69.70%		75.20%
Percentage of Private Education Loans in repayment, Year 4	77.30%		81.10%
Percentage of Private Education Loans in repayment, Year 5	82.20%		85.00%
Percentage of Private Education Loans in repayment, After Year 5	89.00%		89.30%
Percentage of Private Education Loans in repayment	70.50%		73.30%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 1	0.30%		1.00%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 2	1.70%		2.10%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 3	3.70%		3.60%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 4	4.30%		4.10%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 5	4.20%		4.50%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, After Year 5	5.90%		6.80%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment	3.60%		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	1.90%		2.70%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 3	2.90%		2.60%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 4	2.40%		2.30%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 5	2.30%		2.20%
Loans in forbearance as a percentage of loans in repayment and forbearance, After Year 5	2.00%		1.90%
Loans in forbearance as a percentage of loans in repayment and forbearance	1.90%		2.10%
Consumer Portfolio Segment | Student Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans in Repayment, Year 1	$ 2,271,922		$ 2,494,814
Total Private Education Loans in Repayment, Year 2	3,136,672		3,323,327
Total Private Education Loans in Repayment, Year 3	2,250,064		2,209,734
Total Private Education Loans in Repayment, Year 4	1,634,932		1,475,858
Total Private Education Loans in Repayment, Year 5	1,165,916		1,386,889
Total Private Education Loans in Repayment, After Year 5	4,900,749		4,519,192
Total Private Education Loans in repayment	15,360,255		15,409,814
Total Private Education Loans, gross, Year 1	4,145,008		4,490,119
Total Private Education Loans, gross, Year 2	5,363,248		5,088,739
Total Private Education Loans, gross, Year 3	3,229,776		2,937,422
Total Private Education Loans, gross, Year 4	2,113,786		1,819,089
Total Private Education Loans, gross, Year 5	1,419,153		1,632,300
Total Private Education Loans, gross, After Year 5	5,506,495		5,058,175
Total Private Education Loans, gross	21,777,466		21,025,844
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 1	37,794		35,616
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 2	26,654		18,556
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 3	11,375		9,465
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 4	6,512		5,809
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 5	4,325		3,556
Private Education Loans deferred origination costs and unamortized premium/(discount), After Year 5	9,428		8,552
Private Education Loans deferred origination costs and unamortized premium/(discount)	96,088		81,554
Total Private Education Loans, Year 1	4,182,802		4,525,735
Total Private Education Loans, Year 2	5,389,902		5,107,295
Total Private Education Loans, Year 3	3,241,151		2,946,887
Total Private Education Loans, Year 4	2,120,298		1,824,898
Total Private Education Loans, Year 5	1,423,478		1,635,856
Total Private Education Loans, After Year 5	5,515,923		5,066,727
Total Private Education Loans	21,873,554		21,107,398
Private Education Loans allowance for losses, Year 1	(212,169)		(269,642)
Private Education Loans allowance for losses, Year 2	(313,135)		(335,090)
Private Education Loans allowance for losses, Year 3	(235,225)		(194,104)
Private Education Loans allowance for losses, Year 4	(155,737)		(118,755)
Private Education Loans allowance for losses, Year 5	(95,729)		(100,111)
Private Education Loans allowance for losses, After Year 5	(401,626)		(317,403)
Private Education Loans allowance for losses	(1,413,621)		(1,335,105)
Private Education Loans, net, Year 1	3,970,633		4,256,093
Private Education Loans, net, Year 2	5,076,767		4,772,205
Private Education Loans, net, Year 3	3,005,926		2,752,783
Private Education Loans, net, Year 4	1,964,561		1,706,143
Private Education Loans, net, Year 5	1,327,749		1,535,745
Private Education Loans, net, After Year 5	5,114,297		4,749,324
Private Education Loans, net	20,459,933		19,772,293
Consumer Portfolio Segment | Student Loan | Loan delinquent, current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	2,264,429		2,469,817
Year 2	3,081,961		3,254,534
Year 3	2,166,187		2,131,040
Year 4	1,564,016		1,416,069
Year 5	1,116,464		1,323,825
After Year 5	4,613,926		4,213,986
Loan type	14,806,983		14,809,271
Consumer Portfolio Segment | Student Loan | Loan delinquent 30-59 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	5,536		17,599
Year 2	31,829		34,627
Year 3	41,997		37,147
Year 4	34,974		28,020
Year 5	23,902		31,432
After Year 5	147,233		149,926
Loan type	285,471		298,751
Consumer Portfolio Segment | Student Loan | Loan delinquent 60-89 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	1,314		5,720
Year 2	15,492		17,227
Year 3	24,717		20,077
Year 4	20,840		16,614
Year 5	14,467		15,482
After Year 5	72,268		75,897
Loan type	149,098		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	643		1,678
Year 2	7,390		16,939
Year 3	17,163		21,470
Year 4	15,102		15,155
Year 5	11,083		16,150
After Year 5	67,322		79,383
Loan type	118,703		150,775
Loans In-school/grace/deferment | Consumer Portfolio Segment | Student Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	1,868,692		1,976,040
Year 2	2,166,252		1,672,333
Year 3	912,688		669,250
Year 4	438,760		307,781
Year 5	225,235		213,593
After Year 5	504,170		452,994
Loan type	6,115,797		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	4,394		19,265
Total Private Education Loans in forbearance, Year 2	60,324		93,079
Total Private Education Loans in forbearance, Year 3	67,024		58,438
Total Private Education Loans in forbearance, Year 4	40,094		35,450
Total Private Education Loans in forbearance, Year 5	28,002		31,818
Total Private Education Loans in forbearance, After Year 5	101,576		85,989
Total Private Education Loans in forbearance	$ 301,414		$ 324,039