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, 2023	Mar. 31, 2022	Dec. 31, 2022
Accounts, Notes, Loans and Financing Receivable [Line Items]
Modified Loans	$ 94,682		$ 326,116
60 Days or Greater Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Modified Loans	11,624	$ 290
Payment Default(3)	11,404	287
Charge-Offs	4,628	0
Combination - Interest Rate Reduction and Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Modified Loans	10,400	300
Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Modified Loans	1,200	0
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Modified Loans	94,682		$ 326,116
Private Education Loans | 60 Days or Greater Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Modified Loans	11,624	290
Payment Default(3)	11,404	287
Charge-Offs	$ 4,628	$ 0