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Subsequent Events (Details) - USD ($)
12 Months Ended
Feb. 12, 2020
Jun. 12, 2019
Mar. 13, 2019
Sep. 19, 2018
Jun. 20, 2018
Mar. 21, 2018
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Feb. 20, 2020
Feb. 19, 2020
Feb. 20, 2019
Subsequent Event [Line Items]                        
Unsecured debt issued             $ 0 $ 0 $ 197,000,000      
Estimated weighted average life of student loans             5 years 4 months 24 days 5 years 5 months        
Class A and B Notes                        
Subsequent Event [Line Items]                        
Estimated weighted average life of student loans   4 years 4 months 28 days 4 years 3 months 3 days 4 years 3 months 25 days 4 years 4 months 24 days 4 years 5 months 4 days            
Class A and B Notes | London Interbank Offered Rate (LIBOR)                        
Subsequent Event [Line Items]                        
Basis spread on variable rate   1.01% 92.00% 0.77% 0.76% 0.78%            
Commercial Paper | ABCP borrowings                        
Subsequent Event [Line Items]                        
Private education loan funding                       $ 750,000,000
Subsequent Event | Private Education Loans 2020 Term A                        
Subsequent Event [Line Items]                        
Private education loan term accounted for as a secured financing $ 636,000,000                      
Loans sold to third parties $ 636,000,000                      
Ownership interest percentage on asset-backed financing 100.00%                      
Unsecured debt issued $ 634,000,000                      
Subsequent Event | Class A and B Notes                        
Subsequent Event [Line Items]                        
Estimated weighted average life of student loans 4 years 2 months 4 days                      
Subsequent Event | Class A and B Notes | London Interbank Offered Rate (LIBOR)                        
Subsequent Event [Line Items]                        
Basis spread on variable rate 0.88%                      
Subsequent Event | Private Education Loans Sold To Third-Party                        
Subsequent Event [Line Items]                        
Loans sold to third parties                   $ 954,000,000    
Accrued interest included in loans sold                   $ 68,000,000    
Subsequent Event | Commercial Paper | ABCP borrowings                        
Subsequent Event [Line Items]                        
Private education loan funding                     $ 2,000,000,000