Allowance for Credit Losses and Unfunded Loan Commitments - Amortized Cost Basis of Financing Receivables that Subsequently Defaulted (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Mar. 31, 2025	Dec. 31, 2024
Accounts, Notes, Loans and Financing Receivable [Line Items]
Defaulted Amount	$ 143,663		$ 867,295	$ 1,001,410
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Defaulted Amount	145,275		$ 900,913	$ 1,035,055
Private Education Loans | 60 Days or Greater Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Defaulted Amount	40,986	$ 14,496
Period-end Amortized Cost Basis	38,894	14,783
Private Education Loans | Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Defaulted Amount	1,922	1,005
Period-end Amortized Cost Basis	1,803	1,018
Private Education Loans | Combination - Interest Rate Reduction and Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Defaulted Amount	39,064	13,491
Period-end Amortized Cost Basis	$ 37,091	$ 13,765