Allowance for Credit Losses and Unfunded Loan Commitments - Amortized Cost Basis of Financing Receivables that Subsequently Defaulted (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Dec. 31, 2024
Accounts, Notes, Loans and Financing Receivable [Line Items]
Defaulted Amount			$ 274,584		$ 752,880	$ 1,001,410
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Defaulted Amount			278,819		$ 779,318	$ 1,035,055
Private Education Loans \| 60 Days or Greater Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Defaulted Amount	$ 52,536	$ 26,760	83,313	$ 40,128
Period-end Amortized Cost Basis	47,432	27,322	70,160	39,634
Private Education Loans \| Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Defaulted Amount	2,503	1,569	3,887	2,290
Period-end Amortized Cost Basis	2,222	1,592	3,196	2,110
Private Education Loans \| Combination - Interest Rate Reduction and Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Defaulted Amount	50,033	25,191	79,426	37,838
Period-end Amortized Cost Basis	$ 45,210	$ 25,730	$ 66,964	$ 37,524