Allowance for Credit Losses - Loan Status and Aging of Past Due TDR Loans (Details) - Student Loan - Consumer Portfolio Segment - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
TDR loans in repayment and percentage of each status:
Total Private Education Loans in forbearance, Year 1	$ 2,253,860	$ 2,052,521	$ 2,355,395
Troubled Debt Restructured Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR loans in in-school/grace/deferment	80,281	88,750
TDR loans in forbearance	41,464	76,704
TDR loans in repayment and percentage of each status:
Loans	1,057,665	1,274,590
Total Private Education Loans in forbearance, Year 1	$ 935,920	$ 1,109,136
Total TDR loans in repayment	100.00%	100.00%
Total	$ 1,057,665	$ 1,274,590
Loan delinquent, current
TDR loans in repayment and percentage of each status:
Loans	15,005,773	13,898,948	16,315,651
Total	15,005,773	13,898,948	16,315,651
Loan delinquent, current | Troubled Debt Restructured Loans
TDR loans in repayment and percentage of each status:
Loans	$ 832,018	$ 971,880
Loans current, in percentage	88.90%	87.70%
Total	$ 832,018	$ 971,880
Loan delinquent 30-59 days
TDR loans in repayment and percentage of each status:
Loans	308,559	205,528	288,051
Total	308,559	205,528	288,051
Loan delinquent 30-59 days | Troubled Debt Restructured Loans
TDR loans in repayment and percentage of each status:
Loans	$ 48,766	$ 59,249
Loans delinquent (in percentage)	5.20%	5.30%
Total	$ 48,766	$ 59,249
Loan delinquent 60-89 days
TDR loans in repayment and percentage of each status:
Loans	116,947	119,643	121,302
Total	116,947	119,643	121,302
Loan delinquent 60-89 days | Troubled Debt Restructured Loans
TDR loans in repayment and percentage of each status:
Loans	$ 30,575	$ 43,576
Loans delinquent (in percentage)	3.30%	3.90%
Total	$ 30,575	$ 43,576
Loan delinquent 90 days or greater past due
TDR loans in repayment and percentage of each status:
Loans	79,933	80,702	62,666
Total	79,933	80,702	$ 62,666
Loan delinquent 90 days or greater past due | Troubled Debt Restructured Loans
TDR loans in repayment and percentage of each status:
Loans	$ 24,561	$ 34,431
Loans delinquent (in percentage)	2.60%	3.10%
Total	$ 24,561	$ 34,431