Allowance for Credit Losses - Payment Status (Amortized Cost Basis) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2023	Dec. 31, 2022
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment Status (Amortized Cost Basis)	$ 94,682	$ 326,116
Deferment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment Status (Amortized Cost Basis)	412	7,698
Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment Status (Amortized Cost Basis)	92,213	289,134
30-59 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment Status (Amortized Cost Basis)	1,353	13,859
60-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment Status (Amortized Cost Basis)	358	8,809
90 Days or Greater Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment Status (Amortized Cost Basis)	346	6,616
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment Status (Amortized Cost Basis)	94,682	326,116
Private Education Loans | Deferment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment Status (Amortized Cost Basis)	412	7,698
Private Education Loans | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment Status (Amortized Cost Basis)	92,213	289,134
Private Education Loans | 30-59 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment Status (Amortized Cost Basis)	1,353	13,859
Private Education Loans | 60-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment Status (Amortized Cost Basis)	358	8,809
Private Education Loans | 90 Days or Greater Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment Status (Amortized Cost Basis)	$ 346	$ 6,616