Allowance for Credit Losses - Amortized Cost Basis of Financing Receivables with Loan Modifcations (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 12,902	$ 7,679
% of Total Class of Financing Receivable	0.06%	0.04%
Combination - Interest Rate Reduction and Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 81,780	$ 79,597
% of Total Class of Financing Receivable	0.35%	0.37%
Private Education Loans | Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 12,902	$ 7,679
% of Total Class of Financing Receivable	0.06%	0.04%
Private Education Loans | Interest Rate Reduction | Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average contractual rate	12.47%	10.09%
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	$ 81,780	$ 79,597
% of Total Class of Financing Receivable	0.35%	0.37%
Weighted average life of loans	10 years 2 months 8 days	10 years 6 months 3 days
Private Education Loans | Combination - Interest Rate Reduction and Term Extension | Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average contractual rate	12.74%	9.43%
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%