Allowance for Credit Losses - Amortized Cost Basis of Financing Receivables with Loan Modifications (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized Cost Basis	$ 21,159	$ 16,620	$ 28,633	$ 39,263
% of Total Class of Financing Receivable	0.09%	0.07%	0.12%	0.17%
Combination - Interest Rate Reduction and Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized Cost Basis	$ 449,484	$ 90,193	$ 937,723	$ 254,639
% of Total Class of Financing Receivable	1.92%	0.39%	4.01%	1.10%
Private Education Loans | Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized Cost Basis	$ 21,159	$ 16,620	$ 28,633	$ 39,263
% of Total Class of Financing Receivable	0.09%	0.07%	0.12%	0.17%
Private Education Loans | Interest Rate Reduction | Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average contractual rate	13.16%	13.57%	13.18%	13.29%
Private Education Loans | Interest Rate Reduction | Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average contractual rate	3.59%	4.00%	3.59%	4.00%
Private Education Loans | Combination - Interest Rate Reduction and Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized Cost Basis	$ 449,484	$ 90,193	$ 937,723	$ 254,639
% of Total Class of Financing Receivable	1.92%	0.39%	4.01%	1.10%
Weighted average life of loans	9 years 5 months 1 day	10 years 2 months 19 days	9 years 2 months 15 days	10 years 2 months 26 days
Private Education Loans | Combination - Interest Rate Reduction and Term Extension | Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average contractual rate	12.58%	13.12%	12.69%	12.84%
Private Education Loans | Combination - Interest Rate Reduction and Term Extension | Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average contractual rate	3.51%	4.00%	3.60%	4.00%