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Stock-Based Compensation Stock Option Plans (Details) - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Apr. 30, 2021
Apr. 30, 2020
Apr. 30, 2019
Shares      
Options granted (in shares) 0 0 0
Stock options      
Intrinsic value of options      
Intrinsic value of options exercised $ 4,828 $ 833 $ 371
Intrinsic value of options outstanding 24,068 11,779  
Intrinsic value of exercisable options $ 24,068 $ 11,242  
Restated 2006 Plan | Stock options      
Shares      
Outstanding at the beginning of the year (in shares) 334,026 337,026 339,026
Options exercised (in shares) (53,500) (3,000) (2,000)
Outstanding at the end of the year (in shares) 280,526 334,026 337,026
Options exercisable (in shares) 280,526    
Weighted Average Exercise Price      
Outstanding at the beginning of the year (in dollars per share) $ 25.19 $ 25.25 $ 25.29
Options exercised (in dollars per share) 28.45 31.15 32.19
Outstanding at the end of the year (in dollars per share) 24.57 $ 25.19 $ 25.25
Options exercisable (in dollars per share) $ 24.57    
1992 Plan | Stock options      
Shares      
Outstanding at the beginning of the year (in shares) 1,113 14,302 18,302
Options exercised (in shares) (1,113) (13,189) (4,000)
Outstanding at the end of the year (in shares)   1,113 14,302
Weighted Average Exercise Price      
Outstanding at the beginning of the year (in dollars per share) $ 0.59 $ 0.59 $ 0.59
Options exercised (in dollars per share) $ 0.59 0.59 0.59
Outstanding at the end of the year (in dollars per share)   $ 0.59 $ 0.59