Allowance for Credit Losses - Amortized Cost Basis of Financing Receivables with Loan Modifcations (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2023	Dec. 31, 2022
Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 48,637		$ 30,569
% of Total Class of Financing Receivable	0.22%		0.14%
Combination - Interest Rate Reduction and Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 331,889	$ 295,547
% of Total Class of Financing Receivable	1.48%	1.37%
Private Education Loans | Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 48,637		$ 30,569
% of Total Class of Financing Receivable	0.22%		0.14%
Private Education Loans | Interest Rate Reduction | Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average contractual rate			13.37%	11.12%
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	$ 331,889	$ 295,547
% of Total Class of Financing Receivable	1.48%	1.37%
Weighted average life of loans			10 years 2 months 12 days	10 years 4 months 24 days
Private Education Loans | Combination - Interest Rate Reduction and Term Extension | Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average contractual rate			12.92%	10.57%
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%