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, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Dec. 31, 2024
Accounts, Notes, Loans and Financing Receivable [Line Items]
Defaulted Amount			$ 385,492		$ 475,672	$ 1,001,410
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Defaulted Amount			397,363		$ 492,395	$ 1,035,055
Private Education Loans \| 60 Days or Greater Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Defaulted Amount	$ 47,194	$ 87,722	109,695	$ 121,032
Period-end Amortized Cost Basis	43,468	86,969	87,054	123,127
Private Education Loans \| Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Defaulted Amount	2,839	4,635	5,664	5,657
Period-end Amortized Cost Basis	2,702	4,541	4,615	5,904
Private Education Loans \| Combination - Interest Rate Reduction and Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Defaulted Amount	44,355	83,087	104,031	115,375
Period-end Amortized Cost Basis	$ 40,766	$ 82,428	$ 82,439	$ 117,223