Allowance for Credit Losses and Unfunded Loan Commitments - Amortized Cost Basis of Financing Receivables with Loan Modifications (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized Cost Basis	$ 9,767	$ 6,866	$ 24,599	$ 18,272
% of Total Class of Financing Receivable	0.04%	0.03%	0.10%	0.07%
Combination - Interest Rate Reduction and Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized Cost Basis	$ 232,982	$ 142,208	$ 582,804	$ 379,091
% of Total Class of Financing Receivable	0.98%	0.57%	2.45%	1.53%
Private Education Loans \| Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized Cost Basis	$ 9,767	$ 6,866	$ 24,599	$ 18,272
% of Total Class of Financing Receivable	0.04%	0.03%	0.10%	0.07%
Private Education Loans \| Interest Rate Reduction \| Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average contractual rate	12.96%	12.44%	12.80%	12.77%
Private Education Loans \| Interest Rate Reduction \| Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average contractual rate	4.53%	4.46%	4.45%	4.39%
Private Education Loans \| Combination - Interest Rate Reduction and Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized Cost Basis	$ 232,982	$ 142,208	$ 582,804	$ 379,091
% of Total Class of Financing Receivable	0.98%	0.57%	2.45%	1.53%
Weighted average life of loans	9 years 5 months 8 days	9 years 3 months 29 days	9 years 6 months 7 days	9 years 4 months 20 days
Private Education Loans \| Combination - Interest Rate Reduction and Term Extension \| Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average contractual rate	12.12%	12.41%	12.05%	12.32%
Private Education Loans \| Combination - Interest Rate Reduction and Term Extension \| Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average contractual rate	3.05%	4.23%	2.97%	4.08%