Allowance for Credit Losses - Amortized Cost Basis of Financing Receivables with Loan Modifcations (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2022	Sep. 30, 2022
Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 9,750	$ 25,065
% of Total Class of Financing Receivable	0.05%	0.12%
Combination - Interest Rate Reduction and Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 79,765	$ 237,588
% of Total Class of Financing Receivable	0.40%	1.18%
Private Education Loans | Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 9,750	$ 25,065
% of Total Class of Financing Receivable	0.05%	0.12%
Private Education Loans | Interest Rate Reduction | Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average contractual rate	11.31%	10.76%
Private Education Loans | Interest Rate Reduction | Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average contractual rate	4.00%	4.00%
Private Education Loans | Combination - Interest Rate Reduction and Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 79,765	$ 237,588
% of Total Class of Financing Receivable	0.40%	1.18%
Weighted average life of loans	10 years 2 months 26 days	10 years 4 months 17 days
Private Education Loans | Combination - Interest Rate Reduction and Term Extension | Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average contractual rate	10.87%	10.17%
Private Education Loans | Combination - Interest Rate Reduction and Term Extension | Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average contractual rate	4.00%	4.00%