Allowance for Credit Losses - Loan Status and Aging of Past Due TDR Loans (Details) - Student Loan - Consumer Portfolio Segment - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
TDR loans in repayment and percentage of each status:
Total Private Education Loans in repayment, 2020	$ 2,052,521
Total	13,898,948	$ 16,315,651	$ 14,289,705
Troubled Debt Restructured Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR loans in in-school/grace/deferment	88,750	87,749
TDR loans in repayment and percentage of each status:
Total Private Education Loans in forbearance, 2020	76,704	99,054
Loans current	971,880	1,230,954
Total Private Education Loans in repayment, 2020	$ 1,109,136	$ 1,395,163
Loans current, in percentage	87.70%	88.20%
Total TDR loans in repayment(3)	100.00%	100.00%
Total	$ 1,274,590	$ 1,581,966
Loans delinquent 31-60 days
TDR loans in repayment and percentage of each status:
Total	205,528	288,051	231,216
Loans delinquent 31-60 days | Troubled Debt Restructured Loans
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 59,249	$ 85,555
Loans delinquent (in percentage)	5.30%	6.10%
Loans delinquent 61-90 days
TDR loans in repayment and percentage of each status:
Total	$ 119,643	$ 121,302	95,105
Loans delinquent 61-90 days | Troubled Debt Restructured Loans
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 43,576	$ 49,626
Loans delinquent (in percentage)	3.90%	3.60%
Loans delinquent greater than 90 days
TDR loans in repayment and percentage of each status:
Total	$ 80,702	$ 62,666	$ 50,830
Loans delinquent greater than 90 days | Troubled Debt Restructured Loans
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 34,431	$ 29,028
Loans delinquent (in percentage)	3.10%	2.10%