Allowance for Credit Losses - Amortized Cost Basis of Financing Receivables that Subsequently Defaulted (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Accounts, Notes, Loans and Financing Receivable [Line Items]
Modified loans	$ 257,752		$ 380,526
60 Days or Greater Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Modified loans	14,783	$ 11,624
Payment Default	14,496	11,404
Charge-Offs	4,179	4,628
Combination - Interest Rate Reduction and Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Modified loans	13,800	10,400
Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Modified loans	1,000	1,200
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Modified loans	257,752		$ 380,526
Private Education Loans | 60 Days or Greater Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Modified loans	14,783	11,624
Payment Default	14,496	11,404
Charge-Offs	$ 4,179	$ 4,628