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Allowance for Loan Losses - Amortized Cost Basis of Loans Modified to Borrowers with Financial Difficulty (Details) - Private Education Loans [Member] - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2025
Jun. 30, 2024
Jun. 30, 2025
Jun. 30, 2024
Interest Rate Concession [Member]        
Accounts Notes And Loans Receivable [Line Items]        
Loan Modifications, Amortized Cost $ 568 $ 601 $ 1,171 $ 1,143
Loan Modifications, % of Loan Type 3.60% 3.60% 7.40% 6.80%
Loans receivable, weighted average rate of interest 12.30% 13.30% 12.40% 13.30%
Loans receivable, weighted average rate of interest after reduction 5.30% 5.50% 5.40% 5.50%
More Than an Insignificant Payment Delay [Member]        
Accounts Notes And Loans Receivable [Line Items]        
Loan Modifications, Amortized Cost $ 295 $ 314 $ 548 $ 597
Loan Modifications, % of Loan Type 1.90% 1.90% 3.50% 3.60%
Loans receivable, weighted average period added to remaining life of loan 5 months 5 months 7 months 6 months
Combination Rate Reduction and Term Extension [Member]        
Accounts Notes And Loans Receivable [Line Items]        
Loan Modifications, Amortized Cost $ 35 $ 39 $ 75 $ 78
Loan Modifications, % of Loan Type 0.20% 0.20% 0.50% 0.50%
Loans receivable, weighted average rate of interest 12.10% 12.70% 12.00% 12.80%
Loans receivable, weighted average rate of interest after reduction 5.40% 5.30% 5.40% 5.30%
Loans receivable, weighted average period added to remaining life of loan 7 years 7 years 7 years 7 years