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Investments (Details) - USD ($)
$ in Thousands
Jul. 31, 2021
Apr. 30, 2021
Short-term investments:    
Total short-term investments $ 17,953 $ 31,971
Long-term investments:    
Equity Method Investments. 9,178 7,168
Total long-term investments 10,165 12,156
Investment in limited partnership fund    
Long-term investments:    
Equity Method Investments. 9,178 7,168
Available-for-sale securities    
Short-term investments:    
Total short-term investments 17,953 31,971
Long-term investments:    
Total long-term investments 987 4,988
Available-for-sale securities | Municipal securities    
Short-term investments:    
Total short-term investments 12,264 22,245
Long-term investments:    
Total long-term investments 987 988
Available-for-sale securities | U.S. government securities    
Short-term investments:    
Total short-term investments   4,009
Long-term investments:    
Total long-term investments   4,000
Available-for-sale securities | Corporate bonds    
Short-term investments:    
Total short-term investments $ 5,689 $ 5,717