Allowance for Credit Losses - Age Analysis of Past Due Loans Delinquencies (Details) - USD ($) $ in Thousands	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans allowance for losses	$ (1,194,238)	$ (1,081,066)	$ (1,165,335)	$ (1,215,407)	$ (1,160,244)	$ (1,361,723)
Loans held for investment, net	$ 19,622,302		$ 20,341,283
Percentage of Private Education Loans in repayment, 2022	55.30%
Percentage of Private Education Loans in repayment, 2021	61.80%		59.00%
Percentage of Private Education Loans in repayment, 2020	71.80%		67.90%
Percentage of Private Education Loans in repayment, 2019	77.10%		74.30%
Percentage of Private Education Loans in repayment, 2018	81.40%		79.30%
Percentage of Private Education Loans in repayment, 2017 (and prior for current year)	88.20%		82.90%
Percentage of Private Education Loans in repayment, 2016 and prior			87.40%
Percentage of Private Education Loans in repayment	72.40%		74.90%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2022	0.40%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2021	1.60%		0.80%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2020	3.10%		1.40%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2019	4.10%		2.90%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2018	4.70%		3.70%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2017 (and prior for current year)	6.30%		4.30%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2016 and prior			5.20%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment	3.70%		3.30%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2022	0.20%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2021	0.70%		0.50%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2020	1.10%		1.90%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2019	1.40%		2.10%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2018	1.80%		2.30%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2017 (and prior for current year)	2.20%		2.50%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2016 and Prior			2.10%
Loans in forbearance as a percentage of loans in repayment and forbearance	1.40%		1.90%
Student Loan | Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans in repayment, 2022	$ 1,957,690
Total Private Education Loans in repayment, 2021	2,848,743		$ 2,253,860
Total Private Education Loans in repayment, 2020	1,913,533		2,826,916
Total Private Education Loans in repayment, 2019	1,724,135		2,390,841
Total Private Education Loans in repayment, 2018	1,357,703		1,840,790
Total Private Education Loans in repayment, 2017 (and prior for current year)	4,744,752		1,640,870
Total Private Education Loans in repayment, 2016 and prior			4,557,935
Total Private Education Loans in repayment	14,546,556		15,511,212
Total Private Education Loans, gross, 2022	3,538,717
Total Private Education Loans, gross, 2021	4,613,297		3,822,361
Total Private Education Loans, gross, 2020	2,665,608		4,166,283
Total Private Education Loans, gross, 2019	2,237,381		3,216,525
Total Private Education Loans, gross, 2018	1,667,150		2,320,060
Total Private Education Loans, gross, 2017 (and Prior for current year)	5,382,310		1,978,233
Total Private Education Loans, gross, 2016 and Prior			5,213,401
Total Private Education Loans, gross	20,104,463		20,716,863
Private Education Loans deferred origination costs and unamortized premium/(discount), 2022	20,938
Private Education Loans deferred origination costs and unamortized premium/(discount), 2021	16,761		22,169
Private Education Loans deferred origination costs and unamortized premium/(discount), 2020	9,719		16,067
Private Education Loans deferred origination costs and unamortized premium/(discount), 2019	5,935		9,575
Private Education Loans deferred origination costs and unamortized premium/(discount), 2018	3,867		5,918
Private Education Loans deferred origination costs and unamortized premium/(discount), 2017 (and prior for current year)	9,596		4,588
Private Education Loans deferred origination costs and unamortized premium/(discount), 2016 and prior			9,171
Private Education Loans deferred origination costs and unamortized premium/(discount), Total	66,816		67,488
Total Private Education Loans, 2022	3,559,655
Total Private Education Loans, 2021	4,630,058		3,844,530
Total Private Education Loans, 2020	2,675,327		4,182,350
Total Private Education Loans, 2019	2,243,316		3,226,100
Total Private Education Loans, 2018	1,671,017		2,325,978
Total Private Education Loans, 2017 (and prior for current year)	5,391,906		1,982,821
Total Private Education Loans, 2016 and prior			5,222,572
Total Private Education Loans	20,171,279		20,784,351
Private Education Loans allowance for losses, 2022	(260,520)
Private Education Loans allowance for losses, 2021	(296,638)		(248,102)
Private Education Loans allowance for losses, 2020	(153,743)		(239,507)
Private Education Loans allowance for losses, 2019	(135,939)		(195,223)
Private Education Loans allowance for losses, 2018	(91,886)		(129,678)
Private Education Loans allowance for losses, 2017 (and prior for current year)	(251,701)		(99,982)
Private Education Loans allowance for losses, 2016 and prior			(246,485)
Private Education Loans allowance for losses	(1,190,427)		(1,158,977)
Private Education Loans, net, 2022	3,299,135
Private Education Loans, net, 2021	4,333,420		3,596,428
Private Education Loans, net, 2020	2,521,584		3,942,843
Private Education Loans, net, 2019	2,107,377		3,030,877
Private Education Loans, net, 2018	1,579,131		2,196,300
Private Education Loans, net, 2017 (and prior for current year)	5,140,205		1,882,839
Private Education Loans, net, 2016 and prior			4,976,087
Loans held for investment, net	18,980,852		19,625,374
Student Loan | Consumer Portfolio Segment | Loan delinquent, current
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	1,950,830
2021	2,802,675		2,234,876
2020	1,854,054		2,786,646
2019	1,654,125		2,321,728
2018	1,293,502		1,772,651
2017 (and Prior for current year)	4,447,769		1,570,815
2016 and Prior			4,319,057
Total	14,002,955		15,005,773
Student Loan | Consumer Portfolio Segment | Loan delinquent 30-59 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	4,690
2021	25,836		15,148
2020	28,361		29,146
2019	32,374		46,616
2018	28,952		43,197
2017 (and Prior for current year)	135,028		41,695
2016 and Prior			132,757
Total	255,241		308,559
Student Loan | Consumer Portfolio Segment | Loan delinquent 60-89 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	1,542
2021	13,268		3,194
2020	17,592		7,441
2019	19,004		14,044
2018	18,721		14,310
2017 (and Prior for current year)	81,685		16,425
2016 and Prior			61,533
Total	151,812		116,947
Student Loan | Consumer Portfolio Segment | Loan delinquent 90 days or greater past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	628
2021	6,964		642
2020	13,526		3,683
2019	18,632		8,453
2018	16,528		10,632
2017 (and Prior for current year)	80,270		11,935
2016 and Prior			44,588
Total	136,548		79,933
Student Loan | Loans in-school/grace/deferment | Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	1,576,993
2021	1,745,245		1,556,550
2020	730,081		1,283,523
2019	488,712		773,320
2018	284,858		435,657
2017 (and Prior for current year)	530,971		296,008
2016 and Prior			559,356
Total	5,356,860		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, 2022	4,034
Total Private Education Loans in forbearance, 2021	19,309		11,951
Total Private Education Loans in forbearance, 2020	21,994		55,844
Total Private Education Loans in forbearance, 2019	24,534		52,364
Total Private Education Loans in forbearance, 2018	24,589		43,613
Total Private Education Loans in forbearance, 2017 (and prior for current year)	106,587		41,355
Total Private Education Loans in forbearance, 2016 and prior			96,110
Total Private Education Loans in forbearance	$ 201,047		$ 301,237