Allowance for Credit Losses - Payment Status (Amortized Cost Basis) (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2023	Dec. 31, 2022
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment Status (Amortized Cost Basis)	$ 293,902	$ 326,116
Deferment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment Status (Amortized Cost Basis)	5,140	7,698
Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment Status (Amortized Cost Basis)	266,090	289,134
30-59 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment Status (Amortized Cost Basis)	10,137	13,859
60-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment Status (Amortized Cost Basis)	5,565	8,809
90 Days or Greater Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment Status (Amortized Cost Basis)	6,970	6,616
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment Status (Amortized Cost Basis)	293,902	326,116
Private Education Loans | Deferment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment Status (Amortized Cost Basis)	5,140	7,698
Private Education Loans | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment Status (Amortized Cost Basis)	266,090	289,134
Private Education Loans | 30-59 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment Status (Amortized Cost Basis)	10,137	13,859
Private Education Loans | 60-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment Status (Amortized Cost Basis)	5,565	8,809
Private Education Loans | 90 Days or Greater Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment Status (Amortized Cost Basis)	$ 6,970	$ 6,616