Allowance for Credit Losses - Amortized Cost Basis of Financing Receivables with Loan Modifications (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 14,809	$ 17,621	$ 23,995	$ 23,117
% of Total Class of Financing Receivable	0.07%	0.09%	0.11%	0.12%
Combination - Interest Rate Reduction and Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 94,485	$ 96,015	$ 166,882	$ 162,049
% of Total Class of Financing Receivable	0.44%	0.49%	0.78%	0.83%
Private Education Loans | Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 14,809	$ 17,621	$ 23,995	$ 23,117
% of Total Class of Financing Receivable	0.07%	0.09%	0.11%	0.12%
Private Education Loans | Interest Rate Reduction | Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average contractual rate	13.30%	10.38%	13.06%	10.36%
Private Education Loans | Interest Rate Reduction | Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average contractual rate	4.00%	4.00%	4.00%	4.00%
Private Education Loans | Combination - Interest Rate Reduction and Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 94,485	$ 96,015	$ 166,882	$ 162,049
% of Total Class of Financing Receivable	0.44%	0.49%	0.78%	0.83%
Weighted average life of loans	10 years 2 months 23 days	10 years 4 months 2 days	10 years 2 months 23 days	10 years 5 months 1 day
Private Education Loans | Combination - Interest Rate Reduction and Term Extension | Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average contractual rate	12.84%	10.04%	12.69%	9.80%
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%	4.00%	4.00%