Allowance for Credit Losses and Unfunded Loan Commitments - Amortized Cost Basis of Financing Receivables that Subsequently Defaulted (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Dec. 31, 2025
Accounts, Notes, Loans and Financing Receivable [Line Items]
Defaulted Amount			$ 588,444		$ 716,980	$ 514,954
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Defaulted Amount			607,403		$ 741,945	$ 529,634
Private Education Loans \| 60 Days or Greater Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Defaulted Amount	$ 56,274	$ 47,194	104,680	$ 109,695
Period-end Amortized Cost Basis	50,498	43,468	81,217	87,054
Private Education Loans \| Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Defaulted Amount	3,167	2,839	6,141	5,664
Period-end Amortized Cost Basis	2,779	2,702	4,430	4,615
Private Education Loans \| Combination - Interest Rate Reduction and Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Defaulted Amount	53,107	44,355	98,539	104,031
Period-end Amortized Cost Basis	$ 47,719	$ 40,766	$ 76,787	$ 82,439