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Allowance for Credit Losses and Unfunded Loan Commitments - Narrative (Details)
 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2025
USD ($)
Jun. 30, 2024
USD ($)
Jun. 30, 2025
USD ($)
payment
Jun. 30, 2024
USD ($)
Jun. 30, 2025
USD ($)
Dec. 31, 2024
USD ($)
Jul. 01, 2006
Accounts, Notes, Loans and Financing Receivable [Line Items]              
Monthly payment that is smaller than the interest accrued on the loan in that month     $ 25        
Financing receivable allowance period increase (decrease)     $ 143,000,000        
Loan sale reduction to provision   $ 102,751,000   $ 235,955,000      
Criteria for loans to be considered as nonperforming (greater than)     90 days        
Tier 1 of government guarantee (at least) 97.00%   97.00%   97.00%   97.00%
Maximum amount of forbearance granted     12 months        
Number of monthly payments required by a borrower between successive grants of forbearance | payment     12        
Defaulted Amount     $ 274,584,000   $ 752,880,000 $ 1,001,410,000  
Threshold period for payment default     60 days        
Financing receivable, modified in period, subsequently charged off, amount     $ 32,200,000   57,100,000 40,400,000  
Period of loans past due that have accrued interest     90 days        
Private Education Loans | Extended Maturity              
Accounts, Notes, Loans and Financing Receivable [Line Items]              
Financial receivable, excluding accrued interest, modified, accumulated $ 6,100,000   $ 6,100,000   6,100,000    
Financial receivable, excluding accrued interest, modified in period, to total financing receivables, percentage     0.03%        
Financing receivable, modified, weighted average term increase from modification     6 years 10 months 24 days        
Loan delinquent 30-59 days              
Accounts, Notes, Loans and Financing Receivable [Line Items]              
Defaulted Amount     $ 48,282,000   73,440,000 77,446,000  
Loan delinquent, current              
Accounts, Notes, Loans and Financing Receivable [Line Items]              
Defaulted Amount     167,602,000   588,873,000 826,007,000  
Loan delinquent 60-89 days              
Accounts, Notes, Loans and Financing Receivable [Line Items]              
Defaulted Amount     27,306,000   43,057,000 43,484,000  
Loan delinquent 90 days or greater past due              
Accounts, Notes, Loans and Financing Receivable [Line Items]              
Defaulted Amount     31,394,000   47,510,000 54,473,000  
Private Education Loans              
Accounts, Notes, Loans and Financing Receivable [Line Items]              
Loan sale reduction to provision   $ 102,751,000 116,459,000 235,955,000      
Loans sold     2,000,000,000 $ 3,690,000,000      
Defaulted Amount     $ 278,819,000   $ 779,318,000 $ 1,035,055,000  
Threshold period when delinquent loans are written off     120 days        
Financing receivable, modified in period, subsequently charged off, amount $ 17,800,000   $ 32,200,000        
Maximum              
Accounts, Notes, Loans and Financing Receivable [Line Items]              
Increments in which forbearance is granted     2 months        
Minimum              
Accounts, Notes, Loans and Financing Receivable [Line Items]              
Increments in which forbearance is granted     1 month