Allowance for Credit Losses - Age Analysis of Past Due Loans Delinquencies (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans allowance for losses	$ (1,160,244)	$ (1,179,021)	$ (1,361,723)	$ (1,929,323)	$ (1,673,324)	$ (441,912)
Loans held for investment, net	$ 20,115,144		$ 19,183,143
Percentage of Private Education Loans in repayment, 2021	56.40%		59.60%
Percentage of Private Education Loans in repayment, 2020	61.60%		64.70%
Percentage of Private Education Loans in repayment, 2019	67.10%		71.30%
Percentage of Private Education Loans in repayment, 2018	73.40%		75.30%
Percentage of Private Education Loans in repayment, 2017	77.40%		80.00%
Percentage of Private Education Loans in repayment, 2016 and prior	84.70%		84.70%
Percentage of Private Education Loans in repayment	72.40%		72.50%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2021	0.30%		0.50%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2020	0.50%		1.20%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2019	1.30%		2.40%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2018	2.10%		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	3.40%		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	0.20%		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	2.60%		5.00%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2018	3.50%		5.90%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2017	4.30%		5.90%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2016 and Prior	4.00%		4.90%
Loans in forbearance as a percentage of loans in repayment and forbearance	3.00%		4.30%
Student Loan | Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans in repayment, 2021	$ 666,313
Total Private Education Loans in repayment, 2020	2,800,752		$ 2,052,521
Total Private Education Loans in repayment, 2019	2,510,652		2,603,996
Total Private Education Loans in repayment, 2018	1,968,692		2,096,368
Total Private Education Loans in repayment, 2017	1,772,852		1,912,370
Total Private Education Loans in repayment, 2016	5,106,114		1,752,386
Total Private Education Loans in repayment, 2015 and prior			3,887,180
Total Private Education Loans in repayment	14,825,375		14,304,821
Total Private Education Loans, gross, 2021	1,182,226		3,442,765
Total Private Education Loans, gross, 2020	4,549,883		4,026,848
Total Private Education Loans, gross, 2019	3,742,807		2,940,511
Total Private Education Loans, gross, 2018	2,681,876		2,539,794
Total Private Education Loans, gross, 2017	2,290,457		2,189,222
Total Private Education Loans, gross, 2016 and prior	6,030,508		4,590,197
Total Private Education Loans, gross	20,477,757		19,729,337
Private Education Loans deferred origination costs and unamortized premium/(discount), 2021	10,603		21,129
Private Education Loans deferred origination costs and unamortized premium/(discount), 2020	18,946		13,933
Private Education Loans deferred origination costs and unamortized premium/(discount), 2019	12,100		8,671
Private Education Loans deferred origination costs and unamortized premium/(discount), 2018	7,405		6,708
Private Education Loans deferred origination costs and unamortized premium/(discount), 2017	5,693		5,721
Private Education Loans deferred origination costs and unamortized premium/(discount), 2016	11,125		7,313
Private Education Loans deferred origination costs and unamortized premium/(discount), Total	65,872		63,475
Total Private Education Loans, 2021	1,192,829		3,463,894
Total Private Education Loans, 2020	4,568,829		4,040,781
Total Private Education Loans, 2019	3,754,907		2,949,182
Total Private Education Loans, 2018	2,689,281		2,546,502
Total Private Education Loans, 2017	2,296,150		2,194,943
Total Private Education Loans, 2016 and Prior	6,041,633		4,597,510
Total Private Education Loans	20,543,629		19,792,812
Private Education Loans allowance for losses, 2021	(80,501)		(210,875)
Private Education Loans allowance for losses, 2020	(253,285)		(298,776)
Private Education Loans allowance for losses, 2019	(227,307)		(218,136)
Private Education Loans allowance for losses, 2018	(155,165)		(184,265)
Private Education Loans allowance for losses, 2017	(126,301)		(150,150)
Private Education Loans allowance for losses, 2016 and prior	(311,981)		(293,642)
Private Education Loans allowance for losses	(1,154,540)		(1,355,844)
Private Education Loans, net, 2021	1,112,328		3,253,019
Private Education Loans, net, 2020	4,315,544		3,742,005
Private Education Loans, net, 2019	3,527,600		2,731,046
Private Education Loans, net, 2018	2,534,116		2,362,237
Private Education Loans, net, 2017	2,169,849		2,044,793
Private Education Loans, net, 2016 and prior	5,729,652		4,303,868
Loans held for investment, net	19,389,089		18,436,968
Student Loan | Consumer Portfolio Segment | Loan delinquency, current
Accounts, Notes, Loans and Financing Receivable [Line Items]
2021	664,057
2020	2,787,235		2,043,033
2019	2,478,940		2,573,228
2018	1,927,727		2,045,012
2017	1,724,384		1,850,539
2016 and prior	4,932,316		1,685,572
2015 and Prior			3,701,564
Total	14,514,659		13,898,948
Student Loan | Consumer Portfolio Segment | Loan delinquency 30-59 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
2021	1,690
2020	9,085		6,400
2019	18,709		16,983
2018	22,127		26,934
2017	25,059		30,771
2016 and prior	85,868		33,040
2015 and Prior			91,400
Total	162,538		205,528
Student Loan | Consumer Portfolio Segment | Loan delinquency 60-89 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
2021	390
2020	3,380		2,628
2019	8,479		9,143
2018	10,918		15,026
2017	13,229		18,121
2016 and prior	49,055		19,064
2015 and Prior			55,661
Total	85,451		119,643
Student Loan | Consumer Portfolio Segment | Loan delinquency 90 days or greater past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
2021	176
2020	1,052		460
2019	4,524		4,642
2018	7,920		9,396
2017	10,180		12,939
2016 and prior	38,875		14,710
2015 and Prior			38,555
Total	62,727		80,702
Student Loan | Loans In-School/Grace/Deferment | Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
2021	514,519
2020	1,727,389		1,374,085
2019	1,164,034		1,330,175
2018	641,600		733,824
2017	438,616		508,478
2016 and prior	713,049		327,763
2015 and Prior			504,715
Total	5,199,207		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	1,394
Total Private Education Loans in forbearance, 2020	21,742		16,159
Total Private Education Loans in forbearance, 2019	68,121		92,677
Total Private Education Loans in forbearance, 2018	71,584		110,319
Total Private Education Loans in forbearance, 2017	78,989		118,946
Total Private Education Loans in forbearance, 2016 and Prior	211,345		109,073
Total Private Education Loans in forbearance, 2015 and Prior			198,302
Total Private Education Loans in forbearance	453,175		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	479,000		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	249,000		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	$ 23,000		$ 30,000