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	15 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025	Dec. 31, 2025	Jun. 30, 2026
Accounts, Notes, Loans and Financing Receivable [Line Items]
Defaulted Amount			$ 382,832		$ 514,954	$ 630,352
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Defaulted Amount			388,077		$ 529,634	$ 645,433
Private Education Loans \| 60 Days or Greater Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Defaulted Amount	$ 40,745	$ 52,536	61,802	$ 83,313
Period-end Amortized Cost Basis	35,741	47,432	50,184	70,160
Private Education Loans \| Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Defaulted Amount	2,697	2,503	3,825	3,887
Period-end Amortized Cost Basis	2,385	2,222	2,973	3,196
Private Education Loans \| Combination - Interest Rate Reduction and Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Defaulted Amount	38,048	50,033	57,977	79,426
Period-end Amortized Cost Basis	$ 33,356	$ 45,210	$ 47,211	$ 66,964