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Stock Options and Share-Based Compensation - Summary of Stock Option Activity and Weighted Average Exercise Price (Detail) - $ / shares
12 Months Ended
Apr. 30, 2021
Apr. 30, 2020
Apr. 30, 2019
Number of Shares      
Outstanding at beginning of year (in shares) 88,000 104,350  
Canceled (in shares) (3,700) (14,050)  
Exercised (in shares) 0 (2,300)  
Outstanding at end of year (in shares) 84,300 88,000  
Exercisable at end of year (in shares) 84,300 83,400  
Weighted Average Exercise Price      
Outstanding at beginning of year (in dollar per shares) $ 18.56 $ 18.45 $ 18.28
Canceled (in dollar per shares) 15.97 17.78  
Exercised (in dollar per shares) 0 14.98  
Outstanding at end of year (in dollar per shares) 18.56 18.45 $ 18.28
Exercisable at end of year (in dollar per shares) $ 18.56 $ 18.16