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Bonds and Notes Payable - Outstanding Lines of Credit (Details) - FFELP warehouse facilities - Warehouse facilities - USD ($)
Sep. 30, 2019
Mar. 08, 2019
Line of Credit Facility [Line Items]    
Maximum financing amount $ 1,000,000,000  
Amount outstanding 739,448,000  
Amount available 260,552,000  
Advanced as equity support 48,091,000  
NFSLW-I    
Line of Credit Facility [Line Items]    
Maximum financing amount 500,000,000 $ 500,000,000
Amount outstanding 387,510,000  
Amount available 112,490,000  
Advanced as equity support 22,238,000  
NHELP-II    
Line of Credit Facility [Line Items]    
Maximum financing amount 500,000,000  
Amount outstanding 351,938,000  
Amount available 148,062,000  
Advanced as equity support $ 25,853,000