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Investments - Available For Sale Securities (Details) - USD ($)
$ in Thousands
Jul. 31, 2021
Apr. 30, 2021
Available-For-Sale Securities.    
Amortized Cost $ 18,938 $ 36,953
Gross Unrealized Gains 3 9
Gross Unrealized Losses (1) (3)
Total 18,940 36,959
Municipal securities    
Available-For-Sale Securities.    
Amortized Cost 13,249 23,227
Gross Unrealized Gains 3 8
Gross Unrealized Losses (1) (2)
Total 13,251 23,233
U.S. government securities    
Available-For-Sale Securities.    
Amortized Cost   8,008
Gross Unrealized Gains   1
Total   8,009
Corporate bonds    
Available-For-Sale Securities.    
Amortized Cost 5,689 5,718
Gross Unrealized Losses   (1)
Total $ 5,689 $ 5,717