Allowance for Credit Losses - Amortized Cost Basis of Financing Receivables with Loan Modifications (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 4,991	$ 12,902
% of Total Class of Financing Receivable	0.02%	0.06%
Combination - Interest Rate Reduction and Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 252,761	$ 81,780
% of Total Class of Financing Receivable	1.13%	0.35%
Private Education Loans | Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 4,991	$ 12,902
% of Total Class of Financing Receivable	0.02%	0.06%
Private Education Loans | Interest Rate Reduction | Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average contractual rate	13.15%	12.47%
Private Education Loans | Interest Rate Reduction | Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average contractual rate	3.94%	4.00%
Private Education Loans | Combination - Interest Rate Reduction and Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 252,761	$ 81,780
% of Total Class of Financing Receivable	1.13%	0.35%
Weighted average life of loans	8 years 7 months 2 days	10 years 2 months 8 days
Private Education Loans | Combination - Interest Rate Reduction and Term Extension | Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average contractual rate	12.57%	12.74%
Private Education Loans | Combination - Interest Rate Reduction and Term Extension | Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average contractual rate	3.71%	4.00%