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Investments (Details) - USD ($)
$ in Thousands
Oct. 31, 2020
Apr. 30, 2020
Short-term investments:    
Total short-term investments $ 67,137 $ 47,507
Long-term investments:    
Equity Method Investments. 5,335 4,442
Total long-term investments 20,976 15,030
Investment in limited partnership fund    
Long-term investments:    
Equity Method Investments. 5,335 4,442
Available-for-sale securities    
Short-term investments:    
Total short-term investments 67,137 47,507
Long-term investments:    
Total long-term investments 15,641 10,588
Available-for-sale securities | Municipal securities    
Short-term investments:    
Total short-term investments 13,279 5,244
Long-term investments:    
Total long-term investments 5,640 1,592
Available-for-sale securities | U.S. government securities    
Short-term investments:    
Total short-term investments 44,024 33,771
Long-term investments:    
Total long-term investments 10,001 8,996
Available-for-sale securities | Corporate bonds    
Short-term investments:    
Total short-term investments $ 9,834 $ 8,492