Allowance for Credit Losses and Unfunded Loan Commitments - Amortized Cost Basis of Financing Receivables with Loan Modifications (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized Cost Basis	$ 6,768	$ 4,991
% of Total Class of Financing Receivable	0.03%	0.02%
Combination - Interest Rate Reduction and Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized Cost Basis	$ 138,507	$ 252,761
% of Total Class of Financing Receivable	0.57%	1.13%
Private Education Loans | Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized Cost Basis	$ 6,768	$ 4,991
% of Total Class of Financing Receivable	0.03%	0.02%
Private Education Loans | Interest Rate Reduction | Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average contractual rate	13.11%	13.15%
Private Education Loans | Interest Rate Reduction | Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average contractual rate	4.36%	3.94%
Private Education Loans | Combination - Interest Rate Reduction and Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized Cost Basis	$ 138,507	$ 252,761
% of Total Class of Financing Receivable	0.57%	1.13%
Weighted average life of loans	9 years 4 months 6 days	8 years 7 months 2 days
Private Education Loans | Combination - Interest Rate Reduction and Term Extension | Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average contractual rate	12.10%	12.57%
Private Education Loans | Combination - Interest Rate Reduction and Term Extension | Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average contractual rate	4.01%	3.71%