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Investments (Details) - USD ($)
$ in Thousands
Apr. 30, 2021
Apr. 30, 2020
Short-term investments:    
Total short-term investments $ 31,971 $ 47,507
Long-term investments:    
Equity Method Investments. 7,168 4,442
Total long-term investments 12,156 15,030
Investment in limited partnership fund    
Long-term investments:    
Equity Method Investments. 7,168 4,442
Available-for-sale securities    
Short-term investments:    
Total short-term investments 31,971 47,507
Long-term investments:    
Total long-term investments 4,988 10,588
Available-for-sale securities | Municipal securities    
Short-term investments:    
Total short-term investments 22,245 5,244
Long-term investments:    
Total long-term investments 988 1,592
Available-for-sale securities | U.S. government securities    
Short-term investments:    
Total short-term investments 4,009 33,771
Long-term investments:    
Total long-term investments 4,000 8,996
Available-for-sale securities | Corporate bonds    
Short-term investments:    
Total short-term investments $ 5,717 $ 8,492