Allowance for Credit Losses and Unfunded Loan Commitments - Age Analysis of Pas Due Loans Delinquencies - Deferment (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended	12 Months Ended
Jun. 30, 2025	Jun. 30, 2025	Jun. 30, 2025	Dec. 31, 2024
Financing Receivable, Past Due [Line Items]
Financing receivable, modified in period, amount		$ 274,584	$ 752,880	$ 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		$ 32,200	$ 57,100	$ 40,400
Private Education Loans
Financing Receivable, Past Due [Line Items]
Financing receivable, modified in period, amount		278,819	779,318	1,035,055
Financing receivable, modified in period, subsequently charged off, amount	$ 17,800	32,200
Loan Deferment
Financing Receivable, Past Due [Line Items]
Financing receivable, modified in period, amount		4,235	26,438	33,645
Loan delinquent, current
Financing Receivable, Past Due [Line Items]
Financing receivable, modified in period, amount		$ 167,602	$ 588,873	$ 826,007
Financing receivable, modified in period, to total financing receivable, percentage		61.00%	78.00%	83.00%
Loan delinquent 30-59 days
Financing Receivable, Past Due [Line Items]
Financing receivable, modified in period, amount		$ 48,282	$ 73,440	$ 77,446
Financing receivable, modified in period, to total financing receivable, percentage		18.00%	10.00%	8.00%
Loan delinquent 60-89 days
Financing Receivable, Past Due [Line Items]
Financing receivable, modified in period, amount		$ 27,306	$ 43,057	$ 43,484
Financing receivable, modified in period, to total financing receivable, percentage		10.00%	6.00%	4.00%
Loan delinquent 90 days or greater past due
Financing Receivable, Past Due [Line Items]
Financing receivable, modified in period, amount		$ 31,394	$ 47,510	$ 54,473
Financing receivable, modified in period, to total financing receivable, percentage		11.00%	6.00%	5.00%

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