Allowance for Credit Losses - Age Analysis of Pas Due Loans Delinquencies - Deferment (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2024	Sep. 30, 2024	Dec. 31, 2023
Financing Receivable, Past Due [Line Items]
Financing receivable, modified in period, amount	$ 941,410	$ 1,020,863	$ 373,683
Financing receivable, modified in period, to total financing receivable, percentage	100.00%	100.00%	100.00%
Private Education Loans
Financing Receivable, Past Due [Line Items]
Financing receivable, modified in period, amount	$ 966,356	$ 1,049,190	$ 380,526
Loan Deferment
Financing Receivable, Past Due [Line Items]
Financing receivable, modified in period, amount	24,946	28,327	6,843
Loan delinquent, current
Financing Receivable, Past Due [Line Items]
Financing receivable, modified in period, amount	$ 766,273	$ 837,473	$ 334,967
Financing receivable, modified in period, to total financing receivable, percentage	81.00%	82.00%	90.00%
Loan delinquent 30-59 days
Financing Receivable, Past Due [Line Items]
Financing receivable, modified in period, amount	$ 74,279	$ 77,145	$ 17,205
Financing receivable, modified in period, to total financing receivable, percentage	8.00%	8.00%	4.00%
Loan delinquent 60-89 days
Financing Receivable, Past Due [Line Items]
Financing receivable, modified in period, amount	$ 43,208	$ 44,846	$ 7,689
Financing receivable, modified in period, to total financing receivable, percentage	5.00%	4.00%	2.00%
Loan delinquent 90 days or greater past due
Financing Receivable, Past Due [Line Items]
Financing receivable, modified in period, amount	$ 57,650	$ 61,399	$ 13,822
Financing receivable, modified in period, to total financing receivable, percentage	6.00%	6.00%	4.00%

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