Allowance for Credit Losses and Unfunded Loan Commitments - Age Analysis of Pas Due Loans Delinquencies - Deferment (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended		18 Months Ended
	Mar. 31, 2025	Mar. 31, 2025	Dec. 31, 2024	Mar. 31, 2025
Financing Receivable, Past Due [Line Items]
Financing receivable, modified in period, amount	$ 143,663	$ 867,295	$ 1,001,410
Financing receivable, modified in period, to total financing receivable, percentage	100.00%	100.00%	100.00%
Financing receivable, modified in period, subsequently charged off, amount	$ 18,500		$ 40,400	$ 50,000
Private Education Loans
Financing Receivable, Past Due [Line Items]
Financing receivable, modified in period, amount	145,275	$ 900,913	1,035,055
Financing receivable, modified in period, subsequently charged off, amount	18,500
Loan Deferment
Financing Receivable, Past Due [Line Items]
Financing receivable, modified in period, amount	1,612	33,618	33,645
Loan delinquent, current
Financing Receivable, Past Due [Line Items]
Financing receivable, modified in period, amount	$ 48,710	$ 692,649	$ 826,007
Financing receivable, modified in period, to total financing receivable, percentage	34.00%	79.00%	83.00%
Loan delinquent 30-59 days
Financing Receivable, Past Due [Line Items]
Financing receivable, modified in period, amount	$ 43,323	$ 76,207	$ 77,446
Financing receivable, modified in period, to total financing receivable, percentage	30.00%	9.00%	8.00%
Loan delinquent 60-89 days
Financing Receivable, Past Due [Line Items]
Financing receivable, modified in period, amount	$ 26,691	$ 48,976	$ 43,484
Financing receivable, modified in period, to total financing receivable, percentage	19.00%	6.00%	4.00%
Loan delinquent 90 days or greater past due
Financing Receivable, Past Due [Line Items]
Financing receivable, modified in period, amount	$ 24,939	$ 49,463	$ 54,473
Financing receivable, modified in period, to total financing receivable, percentage	17.00%	6.00%	5.00%