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Allowance for Credit Losses - Narrative (Details)
 3 Months Ended 12 Months Ended
Mar. 31, 2024
USD ($)
payment
Dec. 31, 2023
USD ($)
Dec. 31, 2023
USD ($)
Jul. 01, 2006
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Financing receivable allowance period increase (decrease) $ (102,000,000)      
Loan sale reduction to provision $ 133,204,000      
Criteria for loans to be considered as nonperforming (greater than) 90 days      
Tier 1 of government guarantee (at least) 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      
Interest rate reduction   2.00%    
Modified loans $ 257,752,000   $ 380,526,000  
Threshold period for payment default 60 days      
Period of loans past due that have accrued interest 90 days      
Monthly payment that is smaller than the interest accrued on the loan in that month $ 25      
Extended Maturity        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Financing receivable, troubled debt restructuring, subsequent default, number of contracts | payment 0      
Private Education Loans | Extended Maturity        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Financial receivable, excluding accrued interest, modified, accumulated   $ 10,500,000 10,500,000  
Financial receivable, excluding accrued interest, modified in period, to total financing receivables, percentage   0.05%    
Financing receivable, modified, weighted average term increase from modification   6 years 7 months 6 days    
Modified loans $ 200,000      
Financing receivable, troubled debt restructuring, premodification $ 200,000      
Threshold period for payment default 60 days      
Loan delinquent 30-59 days        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Modified loans $ 5,166,000   17,205,000  
Loan delinquent 30-59 days | Private Education Loans | Extended Maturity        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Financial receivable, excluding accrued interest, modified, accumulated 400,000      
Loan delinquent 60-89 days        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Modified loans 2,476,000   7,689,000  
Loan delinquent 60-89 days | Private Education Loans | Extended Maturity        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Financial receivable, excluding accrued interest, modified, accumulated 80,000.00      
Loan delinquent 90 days or greater past due        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Modified loans 2,508,000   13,822,000  
Loan delinquent 90 days or greater past due | Private Education Loans | Extended Maturity        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Financial receivable, excluding accrued interest, modified, accumulated 200,000      
Loan delinquent, current        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Modified loans 244,289,000   334,967,000  
Loan delinquent, current | Private Education Loans | Extended Maturity        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Financial receivable, excluding accrued interest, modified, accumulated 9,800,000      
Private Education Loans        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Loan sale reduction to provision 133,204,000      
Loans sold 2,100,000,000      
Modified loans $ 257,752,000   380,526,000  
Threshold period when delinquent loans are written off 120 days      
Private Education Loans | Loan delinquent 30-59 days        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Modified loans $ 5,166,000   17,205,000  
Private Education Loans | Loan delinquent 60-89 days        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Modified loans 2,476,000   7,689,000  
Private Education Loans | Loan delinquent 90 days or greater past due        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Modified loans 2,508,000   13,822,000  
Private Education Loans | Loan delinquent, current        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Modified loans 244,289,000   $ 334,967,000  
FFELP Loans        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Loan sale reduction to provision $ 0      
Percentage of FFELP loans insured and guaranteed (at least) 97.00%      
Maximum        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Increments in which forbearance is granted 2 months      
Interest rate offered to borrowers facing financial difficulty   8.00% 8.00%  
Temporary interest rate reduction, period   4 years 4 years  
Minimum        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Increments in which forbearance is granted 1 month      
Interest rate offered to borrowers facing financial difficulty   2.00% 2.00%  
Temporary interest rate reduction, period   2 years 2 years