Allowance for Credit Losses - Age Analysis of Past Due Loans Delinquencies (Details) - USD ($) $ in Thousands	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans allowance for losses	$ (1,081,066)	$ (1,227,362)	$ (1,165,335)	$ (1,160,244)	$ (1,179,021)	$ (1,361,723)
Loans held for investment, net	$ 19,201,328		$ 20,341,283
Percentage of Private Education Loans in repayment, 2022	53.40%
Percentage of Private Education Loans in repayment, 2021	60.80%		59.00%
Percentage of Private Education Loans in repayment, 2020	71.20%		67.90%
Percentage of Private Education Loans in repayment, 2019	76.90%		74.30%
Percentage of Private Education Loans in repayment, 2018	81.60%		79.30%
Percentage of Private Education Loans in repayment, 2017 and prior	88.60%		82.90%
Percentage of Private Education Loans in repayment, 2016 and prior			87.40%
Percentage of Private Education Loans in repayment	74.40%		74.90%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2022	0.70%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2021	1.20%		0.80%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2020	2.40%		1.40%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2019	3.60%		2.90%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2018	4.50%		3.70%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2017 and prior	6.00%		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	0.50%		0.50%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2020	0.90%		1.90%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2019	1.10%		2.10%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2018	1.50%		2.30%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2017 and Prior	1.90%		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	1.30%		1.90%
Student Loan | Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans in repayment, 2022	$ 683,933
Total Private Education Loans in repayment, 2021	2,951,456		$ 2,253,860
Total Private Education Loans in repayment, 2020	2,221,088		2,826,916
Total Private Education Loans in repayment, 2019	1,933,334		2,390,841
Total Private Education Loans in repayment, 2018	1,509,043		1,840,790
Total Private Education Loans in repayment, 2017	5,226,405		1,640,870
Total Private Education Loans in repayment, 2016 and prior			4,557,935
Total Private Education Loans in repayment	14,525,259		15,511,212
Total Private Education Loans, gross, 2022	1,279,752
Total Private Education Loans, gross, 2021	4,854,861		3,822,361
Total Private Education Loans, gross, 2020	3,120,690		4,166,283
Total Private Education Loans, gross, 2019	2,513,490		3,216,525
Total Private Education Loans, gross, 2018	1,848,355		2,320,060
Total Private Education Loans, gross, 2017 and prior	5,902,037		1,978,233
Total Private Education Loans, gross, 2016 and prior			5,213,401
Total Private Education Loans, gross	19,519,185		20,716,863
Private Education Loans deferred origination costs and unamortized premium/(discount), 2022	14,647
Private Education Loans deferred origination costs and unamortized premium/(discount), 2021	18,565		22,169
Private Education Loans deferred origination costs and unamortized premium/(discount), 2020	11,502		16,067
Private Education Loans deferred origination costs and unamortized premium/(discount), 2019	6,923		9,575
Private Education Loans deferred origination costs and unamortized premium/(discount), 2018	4,421		5,918
Private Education Loans deferred origination costs and unamortized premium/(discount), 2017 and prior	10,751		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	66,809		67,488
Total Private Education Loans, 2022	1,294,399
Total Private Education Loans, 2021	4,873,426		3,844,530
Total Private Education Loans, 2020	3,132,192		4,182,350
Total Private Education Loans, 2019	2,520,413		3,226,100
Total Private Education Loans, 2018	1,852,776		2,325,978
Total Private Education Loans, 2017 and Prior	5,912,788		1,982,821
Total Private Education Loans, 2016 and Prior			5,222,572
Total Private Education Loans	19,585,994		20,784,351
Private Education Loans allowance for losses, 2022	(118,056)
Private Education Loans allowance for losses, 2021	(287,653)		(248,102)
Private Education Loans allowance for losses, 2020	(168,698)		(239,507)
Private Education Loans allowance for losses, 2019	(144,986)		(195,223)
Private Education Loans allowance for losses, 2018	(95,567)		(129,678)
Private Education Loans allowance for losses, 2017 and prior	(259,784)		(99,982)
Private Education Loans allowance for losses, 2016 and prior			(246,485)
Private Education Loans allowance for losses	(1,074,744)		(1,158,977)
Private Education Loans, net, 2022	1,176,343
Private Education Loans, net, 2021	4,585,773		3,596,428
Private Education Loans, net, 2020	2,963,494		3,942,843
Private Education Loans, net, 2019	2,375,427		3,030,877
Private Education Loans, net, 2018	1,757,209		2,196,300
Private Education Loans, net, 2017 and prior	5,653,004		1,882,839
Private Education Loans, net, 2016 and prior			4,976,087
Loans held for investment, net	18,511,250		19,625,374
Student Loan | Consumer Portfolio Segment | Loan delinquent, current
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	679,111
2021	2,917,375		2,234,876
2020	2,167,862		2,786,646
2019	1,864,069		2,321,728
2018	1,440,757		1,772,651
2017 and Prior	4,911,636		1,570,815
2016 and Prior			4,319,057
Total	13,980,810		15,005,773
Student Loan | Consumer Portfolio Segment | Loan delinquent 30-59 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	3,513
2021	21,115		15,148
2020	25,780		29,146
2019	31,433		46,616
2018	30,911		43,197
2017 and Prior	142,525		41,695
2016 and Prior			132,757
Total	255,277		308,559
Student Loan | Consumer Portfolio Segment | Loan delinquent 60-89 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	761
2021	8,512		3,194
2020	15,626		7,441
2019	21,292		14,044
2018	22,816		14,310
2017 and Prior	100,649		16,425
2016 and Prior			61,533
Total	169,656		116,947
Student Loan | Consumer Portfolio Segment | Loan delinquent 90 days or greater past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	548
2021	4,454		642
2020	11,820		3,683
2019	16,540		8,453
2018	14,559		10,632
2017 and Prior	71,595		11,935
2016 and Prior			44,588
Total	119,516		79,933
Student Loan | Loans in-school/grace/deferment | Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	594,307
2021	1,887,781		1,556,550
2020	878,657		1,283,523
2019	557,968		773,320
2018	315,998		435,657
2017 and Prior	575,312		296,008
2016 and Prior			559,356
Total	4,810,023		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	1,512
Total Private Education Loans in forbearance, 2021	15,624		11,951
Total Private Education Loans in forbearance, 2020	20,945		55,844
Total Private Education Loans in forbearance, 2019	22,188		52,364
Total Private Education Loans in forbearance, 2018	23,314		43,613
Total Private Education Loans in forbearance, 2017 and Prior	100,320		41,355
Total Private Education Loans in forbearance, 2016 and Prior			96,110
Total Private Education Loans in forbearance	$ 183,903		$ 301,237