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Borrowings - Narrative (Details) - USD ($)
3 Months Ended 12 Months Ended
Mar. 15, 2023
Mar. 31, 2023
Mar. 31, 2023
Mar. 31, 2023
Dec. 31, 2022
May 17, 2022
Line of Credit Facility [Line Items]            
Estimated weighted average life of student loans   5 years     5 years  
Vertical risk retention interest     5.00% 5.00%    
Uncommitted federal funds   $ 125,000,000 $ 125,000,000 $ 125,000,000    
Lendable value of collateral   2,100,000,000 2,100,000,000 2,100,000,000 $ 2,200,000,000  
ABCP borrowings | Secured Borrowings            
Line of Credit Facility [Line Items]            
Secured borrowing facility maximum borrowing capacity           $ 2,000,000,000
Residual interest percentage of facility trust           100.00%
Secured borrowings outstanding   0 0 0 $ 0  
2023-A            
Line of Credit Facility [Line Items]            
Private education loan term accounted for as a secured financing $ 579,000,000          
Loans sold to third parties $ 579,000,000          
Ownership interest percentage on asset-backed financing 100.00%          
Unsecured debt issued $ 572,000,000          
Estimated weighted average life of student loans 5 years 21 days          
Loans pledged as collateral   639,000,000 639,000,000 639,000,000    
Principal amount of collateral   598,000,000 598,000,000 598,000,000    
Capitalized interest   $ 41,000,000 $ 41,000,000 $ 41,000,000    
2023-A | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate            
Line of Credit Facility [Line Items]            
Basis spread on variable rate 1.53%