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	15 Months Ended
Jun. 30, 2026	Jun. 30, 2026	Dec. 31, 2025	Jun. 30, 2026
Financing Receivable, Past Due [Line Items]
Financing receivable, modified in period, amount		$ 382,832	$ 514,954	$ 630,352
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		$ 28,200	$ 39,100	$ 42,100
Private Education Loans
Financing Receivable, Past Due [Line Items]
Financing receivable, modified in period, amount		388,077	529,634	645,433
Financing receivable, modified in period, subsequently charged off, amount	$ 16,000	28,200
Loan Deferment
Financing Receivable, Past Due [Line Items]
Financing receivable, modified in period, amount		5,245	14,680	15,081
Loan delinquent, current
Financing Receivable, Past Due [Line Items]
Financing receivable, modified in period, amount		$ 270,708	$ 358,054	$ 479,762
Financing receivable, modified in period, to total financing receivable, percentage		71.00%	70.00%	76.00%
Loan delinquent 30-59 days
Financing Receivable, Past Due [Line Items]
Financing receivable, modified in period, amount		$ 46,125	$ 68,823	$ 63,325
Financing receivable, modified in period, to total financing receivable, percentage		12.00%	13.00%	10.00%
Loan delinquent 60-89 days
Financing Receivable, Past Due [Line Items]
Financing receivable, modified in period, amount		$ 28,841	$ 41,592	$ 39,100
Financing receivable, modified in period, to total financing receivable, percentage		7.00%	8.00%	6.00%
Loan delinquent 90 days or greater past due
Financing Receivable, Past Due [Line Items]
Financing receivable, modified in period, amount		$ 37,158	$ 46,485	$ 48,165
Financing receivable, modified in period, to total financing receivable, percentage		10.00%	9.00%	8.00%

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