Allowance for Credit Losses - Age Analysis of Past Due Loans Delinquencies (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans allowance for losses	$ (1,227,362)	$ (1,165,335)	$ (1,179,021)	$ (1,361,723)
Loans held for investment, net	$ 21,291,675	$ 20,341,283
Percentage of Private Education Loans in repayment, 2022	53.40%
Percentage of Private Education Loans in repayment, 2021	59.50%	59.00%
Percentage of Private Education Loans in repayment, 2020	69.10%	67.90%
Percentage of Private Education Loans in repayment, 2019	75.20%	74.30%
Percentage of Private Education Loans in repayment, 2018	80.40%	79.30%
Percentage of Private Education Loans in repayment, 2017 and prior	87.70%	82.90%
Percentage of Private Education Loans in repayment, 2016 and prior		87.40%
Percentage of Private Education Loans in repayment	74.10%	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	0.90%	0.80%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2020	2.20%	1.40%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2019	3.50%	2.90%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2018	4.30%	3.70%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2017 and prior	5.40%	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	3.50%	3.30%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2022	0.10%
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	1.30%	1.90%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2019	1.50%	2.10%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2018	1.70%	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	1.40%	1.90%
Student Loan | Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans in repayment, 2022	$ 417,417
Total Private Education Loans in repayment, 2021	3,000,284	$ 2,253,860
Total Private Education Loans in repayment, 2020	2,751,187	2,826,916
Total Private Education Loans in repayment, 2019	2,285,817	2,390,841
Total Private Education Loans in repayment, 2018	1,749,113	1,840,790
Total Private Education Loans in repayment, 2017	5,891,339	1,640,870
Total Private Education Loans in repayment, 2016 and prior		4,557,935
Total Private Education Loans in repayment	16,095,157	15,511,212
Total Private Education Loans, gross, 2022	781,612
Total Private Education Loans, gross, 2021	5,040,792	3,822,361
Total Private Education Loans, gross, 2020	3,983,606	4,166,283
Total Private Education Loans, gross, 2019	3,038,337	3,216,525
Total Private Education Loans, gross, 2018	2,174,288	2,320,060
Total Private Education Loans, gross, 2017 and prior	6,716,734	1,978,233
Total Private Education Loans, gross, 2016 and prior		5,213,401
Total Private Education Loans, gross	21,735,369	20,716,863
Private Education Loans deferred origination costs and unamortized premium/(discount), 2022	9,615
Private Education Loans deferred origination costs and unamortized premium/(discount), 2021	20,732	22,169
Private Education Loans deferred origination costs and unamortized premium/(discount), 2020	14,883	16,067
Private Education Loans deferred origination costs and unamortized premium/(discount), 2019	8,753	9,575
Private Education Loans deferred origination costs and unamortized premium/(discount), 2018	5,380	5,918
Private Education Loans deferred origination costs and unamortized premium/(discount), 2017 and prior	12,544	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	71,907	67,488
Total Private Education Loans, 2022	791,227
Total Private Education Loans, 2021	5,061,524	3,844,530
Total Private Education Loans, 2020	3,998,489	4,182,350
Total Private Education Loans, 2019	3,047,090	3,226,100
Total Private Education Loans, 2018	2,179,668	2,325,978
Total Private Education Loans, 2017 and Prior	6,729,278	1,982,821
Total Private Education Loans, 2016 and Prior		5,222,572
Total Private Education Loans	21,807,276	20,784,351
Private Education Loans allowance for losses, 2022	(54,769)
Private Education Loans allowance for losses, 2021	(308,664)	(248,102)
Private Education Loans allowance for losses, 2020	(230,000)	(239,507)
Private Education Loans allowance for losses, 2019	(187,181)	(195,223)
Private Education Loans allowance for losses, 2018	(122,741)	(129,678)
Private Education Loans allowance for losses, 2017 and prior	(317,698)	(99,982)
Private Education Loans allowance for losses, 2016 and prior		(246,485)
Private Education Loans allowance for losses	(1,221,053)	(1,158,977)
Private Education Loans, net, 2022	736,458
Private Education Loans, net, 2021	4,752,860	3,596,428
Private Education Loans, net, 2020	3,768,489	3,942,843
Private Education Loans, net, 2019	2,859,909	3,030,877
Private Education Loans, net, 2018	2,056,927	2,196,300
Private Education Loans, net, 2017 and prior	6,411,580	1,882,839
Private Education Loans, net, 2016 and prior		4,976,087
Loans held for investment, net	20,586,223	19,625,374
Student Loan | Consumer Portfolio Segment | Loan delinquent, current
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	415,657
2021	2,972,685	2,234,876
2020	2,690,157	2,786,646
2019	2,205,417	2,321,728
2018	1,674,303	1,772,651
2017 and Prior	5,571,861	1,570,815
2016 and Prior		4,319,057
Total	15,530,080	15,005,773
Student Loan | Consumer Portfolio Segment | Loan delinquent 30-59 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	1,760
2021	17,435	15,148
2020	28,523	29,146
2019	34,956	46,616
2018	33,362	43,197
2017 and Prior	144,499	41,695
2016 and Prior		132,757
Total	260,535	308,559
Student Loan | Consumer Portfolio Segment | Loan delinquent 60-89 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	0
2021	6,557	3,194
2020	18,618	7,441
2019	25,590	14,044
2018	23,023	14,310
2017 and Prior	95,272	16,425
2016 and Prior		61,533
Total	169,060	116,947
Student Loan | Consumer Portfolio Segment | Loan delinquent 90 days or greater past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	0
2021	3,607	642
2020	13,889	3,683
2019	19,854	8,453
2018	18,425	10,632
2017 and Prior	79,707	11,935
2016 and Prior		44,588
Total	135,482	79,933
Student Loan | Loans in-school/grace/deferment | Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	363,754
2021	2,025,385	1,556,550
2020	1,195,182	1,283,523
2019	717,261	773,320
2018	394,590	435,657
2017 and Prior	709,780	296,008
2016 and Prior		559,356
Total	5,405,952	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	441
Total Private Education Loans in forbearance, 2021	15,123	11,951
Total Private Education Loans in forbearance, 2020	37,237	55,844
Total Private Education Loans in forbearance, 2019	35,259	52,364
Total Private Education Loans in forbearance, 2018	30,585	43,613
Total Private Education Loans in forbearance, 2017 and Prior	115,615	41,355
Total Private Education Loans in forbearance, 2016 and Prior		96,110
Total Private Education Loans in forbearance	$ 234,260	$ 301,237