Allowance for Credit Losses - Payment Status (Amortized Cost Basis) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment Status (Amortized Cost Basis)	$ 513,371	$ 380,526
Deferment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment Status (Amortized Cost Basis)	7,527	6,843
Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment Status (Amortized Cost Basis)	468,384	334,967
30-59 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment Status (Amortized Cost Basis)	17,356	17,205
60-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment Status (Amortized Cost Basis)	8,605	7,689
90 Days or Greater Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment Status (Amortized Cost Basis)	11,499	13,822
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment Status (Amortized Cost Basis)	513,371	380,526
Private Education Loans | Deferment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment Status (Amortized Cost Basis)	7,527	6,843
Private Education Loans | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment Status (Amortized Cost Basis)	468,384	334,967
Private Education Loans | 30-59 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment Status (Amortized Cost Basis)	17,356	17,205
Private Education Loans | 60-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment Status (Amortized Cost Basis)	8,605	7,689
Private Education Loans | 90 Days or Greater Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment Status (Amortized Cost Basis)	$ 11,499	$ 13,822