Allowance for Credit Losses - Amortized Cost Basis of Financing Receivables that Subsequently Defaulted (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Accounts, Notes, Loans and Financing Receivable [Line Items]
Modified Loans			$ 293,902		$ 326,116
60 Days or Greater Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Modified Loans	$ 14,546	$ 9,467	26,449	$ 12,660
Payment Default	14,129	9,289	27,672	12,463
Charge-Offs	4,534	1,801	6,428	1,861
Combination - Interest Rate Reduction and Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Modified Loans	12,400	8,500	23,000	11,400
Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Modified Loans	2,100	1,000	3,400	1,200
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Modified Loans			293,902		$ 326,116
Private Education Loans | 60 Days or Greater Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Modified Loans	14,546	9,467	26,449	12,660
Payment Default	14,129	9,289	27,672	12,463
Charge-Offs	$ 4,534	$ 1,801	$ 6,428	$ 1,861