Allowance for Credit Losses and Unfunded Loan Commitments - Age Analysis of Pas Due Loans Delinquencies - Deferment (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2026	Sep. 30, 2026	Sep. 30, 2026	Dec. 31, 2025
Financing Receivable, Past Due [Line Items]
Financing receivable, modified in period, amount		$ 588,444	$ 716,980	$ 514,954
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		$ 40,700	$ 45,200	$ 39,100
Private Education Loans
Financing Receivable, Past Due [Line Items]
Financing receivable, modified in period, amount		607,403	741,945	529,634
Financing receivable, modified in period, subsequently charged off, amount	$ 18,000	40,700
Loan Deferment
Financing Receivable, Past Due [Line Items]
Financing receivable, modified in period, amount		18,959	24,965	14,680
Loan delinquent, current
Financing Receivable, Past Due [Line Items]
Financing receivable, modified in period, amount		$ 430,181	$ 539,819	$ 358,054
Financing receivable, modified in period, to total financing receivable, percentage		74.00%	75.00%	70.00%
Loan delinquent 30-59 days
Financing Receivable, Past Due [Line Items]
Financing receivable, modified in period, amount		$ 77,875	$ 86,317	$ 68,823
Financing receivable, modified in period, to total financing receivable, percentage		13.00%	12.00%	13.00%
Loan delinquent 60-89 days
Financing Receivable, Past Due [Line Items]
Financing receivable, modified in period, amount		$ 42,636	$ 47,916	$ 41,592
Financing receivable, modified in period, to total financing receivable, percentage		7.00%	7.00%	8.00%
Loan delinquent 90 days or greater past due
Financing Receivable, Past Due [Line Items]
Financing receivable, modified in period, amount		$ 37,752	$ 42,928	$ 46,485
Financing receivable, modified in period, to total financing receivable, percentage		6.00%	6.00%	9.00%

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