Allowance for Credit Losses - Age Analysis of Past Due Loans Delinquencies (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans allowance for losses	$ (1,179,021)	$ (1,361,723)	$ (1,673,324)	$ (441,912)
Loans held for investment, net	$ 20,368,287	$ 19,183,143
Percentage of Private Education Loans in repayment, 2021	55.80%	59.60%
Percentage of Private Education Loans in repayment, 2020	60.20%	64.70%
Percentage of Private Education Loans in repayment, 2019	65.20%	71.30%
Percentage of Private Education Loans in repayment, 2018	71.40%	75.30%
Percentage of Private Education Loans in repayment, 2017	75.20%	80.00%
Percentage of Private Education Loans in repayment, 2016 and prior	83.10%	84.70%
Percentage of Private Education Loans in repayment	71.20%	72.50%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2021	0.10%	0.50%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2020	0.30%	1.20%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2019	1.00%	2.40%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2018	1.90%	3.20%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2017	2.70%	3.80%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2016 and prior	3.50%	4.80%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment	2.10%	2.80%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2021	0.30%	0.80%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2020	0.80%	3.40%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2019	3.40%	5.00%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2018	4.50%	5.90%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2017	5.40%	5.90%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2016 and Prior	4.70%	4.90%
Loans in forbearance as a percentage of loans in repayment and forbearance	3.70%	4.30%
Student Loan | Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
2021	$ 418,103
2020	2,747,747	$ 2,043,033
2019	2,491,380	2,573,228
2018	1,955,468	2,045,012
2017	1,755,863	1,850,539
2016 and prior	5,102,866	1,685,572
2015 and Prior		3,701,564
Total	14,471,427	13,898,948
Total Private Education Loans in repayment, 2021	418,469
Total Private Education Loans in repayment, 2020	2,757,024	2,052,521
Total Private Education Loans in repayment, 2019	2,517,627	2,603,996
Total Private Education Loans in repayment, 2018	1,993,924	2,096,368
Total Private Education Loans in repayment, 2017	1,803,993	1,912,370
Total Private Education Loans in repayment, 2016	5,286,902	1,752,386
Total Private Education Loans in repayment, 2015 and prior		3,887,180
Total Private Education Loans in repayment	14,777,939	14,304,821
Total Private Education Loans, gross, 2021	750,072	3,442,765
Total Private Education Loans, gross, 2020	4,577,417	4,026,848
Total Private Education Loans, gross, 2019	3,862,277	2,940,511
Total Private Education Loans, gross, 2018	2,791,354	2,539,794
Total Private Education Loans, gross, 2017	2,397,399	2,189,222
Total Private Education Loans, gross, 2016 and prior	6,363,321	4,590,197
Total Private Education Loans, gross	20,741,840	19,729,337
Private Education Loans deferred origination costs and unamortized premium/(discount), 2021	5,487	21,129
Private Education Loans deferred origination costs and unamortized premium/(discount), 2020	19,922	13,933
Private Education Loans deferred origination costs and unamortized premium/(discount), 2019	12,925	8,671
Private Education Loans deferred origination costs and unamortized premium/(discount), 2018	7,977	6,708
Private Education Loans deferred origination costs and unamortized premium/(discount), 2017	6,153	5,721
Private Education Loans deferred origination costs and unamortized premium/(discount), 2016	12,004	7,313
Private Education Loans deferred origination costs and unamortized premium/(discount), Total	64,468	63,475
Total Private Education Loans, 2021	755,559	3,463,894
Total Private Education Loans, 2020	4,597,339	4,040,781
Total Private Education Loans, 2019	3,875,202	2,949,182
Total Private Education Loans, 2018	2,799,331	2,546,502
Total Private Education Loans, 2017	2,403,552	2,194,943
Total Private Education Loans, 2016 and Prior	6,375,325	4,597,510
Total Private Education Loans	20,806,308	19,792,812
Private Education Loans allowance for losses, 2021	(35,515)	(210,875)
Private Education Loans allowance for losses, 2020	(252,247)	(298,776)
Private Education Loans allowance for losses, 2019	(239,917)	(218,136)
Private Education Loans allowance for losses, 2018	(169,296)	(184,265)
Private Education Loans allowance for losses, 2017	(140,894)	(150,150)
Private Education Loans allowance for losses, 2016 and prior	(335,506)	(293,642)
Private Education Loans allowance for losses	(1,173,375)	(1,355,844)
Private Education Loans, net, 2021	720,044	3,253,019
Private Education Loans, net, 2020	4,345,092	3,742,005
Private Education Loans, net, 2019	3,635,285	2,731,046
Private Education Loans, net, 2018	2,630,035	2,362,237
Private Education Loans, net, 2017	2,262,658	2,044,793
Private Education Loans, net, 2016 and prior	6,039,819	4,303,868
Loans held for investment, net	19,632,933	18,436,968
Student Loan | Consumer Portfolio Segment | Loan delinquency 30-59 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
2021	366
2020	6,587	6,400
2019	14,015	16,983
2018	19,145	26,934
2017	24,043	30,771
2016 and prior	88,587	33,040
2015 and Prior		91,400
Total	152,743	205,528
Student Loan | Consumer Portfolio Segment | Loan delinquency 60-89 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
2021	0
2020	1,747	2,628
2019	7,654	9,143
2018	11,064	15,026
2017	13,166	18,121
2016 and prior	52,759	19,064
2015 and Prior		55,661
Total	86,390	119,643
Student Loan | Consumer Portfolio Segment | Loan delinquency 90 days or greater past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
2021	0
2020	943	460
2019	4,578	4,642
2018	8,247	9,396
2017	10,921	12,939
2016 and prior	42,690	14,710
2015 and Prior		38,555
Total	67,379	80,702
Student Loan | Loans In-School/Grace/Deferment | Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
2021	330,417
2020	1,798,922	1,374,085
2019	1,255,241	1,330,175
2018	703,917	733,824
2017	489,599	508,478
2016 and prior	815,578	327,763
2015 and Prior		504,715
Total	5,393,674	4,779,040
Student Loan | Loans In Forbearance | Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans in forbearance, 2021	1,186
Total Private Education Loans in forbearance, 2020	21,471	16,159
Total Private Education Loans in forbearance, 2019	89,409	92,677
Total Private Education Loans in forbearance, 2018	93,513	110,319
Total Private Education Loans in forbearance, 2017	103,807	118,946
Total Private Education Loans in forbearance, 2016 and Prior	260,841	109,073
Total Private Education Loans in forbearance, 2015 and Prior		198,302
Total Private Education Loans in forbearance	570,227	645,476
Student Loan, Borrowers Who Did Not Return To School And Received Extenstion | Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans in repayment, 2021	482,000	609,000
Student Loan, Borrowers Who Did Not Return To School And Received Extenstion | Loans In-School/Grace/Deferment | Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
2021	273,000	401,000
Student Loan, Borrowers Who Did Not Return To School And Received Extenstion | Loans In Forbearance | Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans in forbearance, 2021	$ 29,000	$ 30,000