Allowance for Credit Losses - Loan Status and Aging of Past Due TDR Loans (Details) - Student Loan - Consumer Portfolio Segment - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
TDR loans in repayment and percentage of each status:
Total Private Education Loans in repayment, 2021	$ 1,834,164
Troubled Debt Restructured Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR loans in in-school/grace/deferment	90,417	$ 88,750
TDR loans in forbearance	48,892	76,704
TDR loans in repayment and percentage of each status:
Loans	1,148,282	1,274,590
Total Private Education Loans in repayment, 2021	$ 1,008,973	$ 1,109,136
Total TDR loans in repayment	100.00%	100.00%
Total	$ 1,148,282	$ 1,274,590
Loan delinquent, current
TDR loans in repayment and percentage of each status:
Loans	15,115,541	13,898,948
Total	15,115,541	13,898,948
Loan delinquent, current \| Troubled Debt Restructured Loans
TDR loans in repayment and percentage of each status:
Loans	$ 917,756	$ 971,880
Loans current	91.00%	87.70%
Total	$ 917,756	$ 971,880
Loan delinquent 30-59 days
TDR loans in repayment and percentage of each status:
Loans	199,942	205,528
Total	199,942	205,528
Loan delinquent 30-59 days \| Troubled Debt Restructured Loans
TDR loans in repayment and percentage of each status:
Loans	$ 43,766	$ 59,249
Loans delinquent	4.30%	5.30%
Total	$ 43,766	$ 59,249
Loan delinquent 60-89 days
TDR loans in repayment and percentage of each status:
Loans	101,512	119,643
Total	101,512	119,643
Loan delinquent 60-89 days \| Troubled Debt Restructured Loans
TDR loans in repayment and percentage of each status:
Loans	$ 24,984	$ 43,576
Loans delinquent	2.50%	3.90%
Total	$ 24,984	$ 43,576
Loan delinquent 90 days or greater past due
TDR loans in repayment and percentage of each status:
Loans	73,137	80,702
Total	73,137	80,702
Loan delinquent 90 days or greater past due \| Troubled Debt Restructured Loans
TDR loans in repayment and percentage of each status:
Loans	$ 22,467	$ 34,431
Loans delinquent	2.20%	3.10%
Total	$ 22,467	$ 34,431

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