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Note K - Stock-based Compensation Plans - Stock Option Activity (Details) - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Apr. 30, 2017
Apr. 30, 2016
Apr. 30, 2015
Apr. 30, 2014
Proceeds on Exercise, Exercised $ (4,363) $ (400) $ (4,143)  
Proceeds on Exercise, Granted 1,301 16,471 3,997  
Proceeds on Exercise $ 34,460 $ 39,081 $ 23,608 $ 24,294
Number of Shares (in shares) 1,278,250 981,750 1,117,000  
Exercise Price (in dollars per share) $ 30.57 $ 24.05 $ 21.75  
Number of Shares, Granted (in shares) 45,000 338,750 89,000  
Exercise Price, Granted (in dollars per share) $ 28.90 $ 48.62 $ 44.91  
Number of Shares, Exercised (in shares) (264,500) (30,750) (212,250)  
Exercise Price, Exercised (in dollars per share) $ 17.92 $ 13 $ 19.52  
Number of Shares, Cancelled (in shares) (30,250) (11,500) (12,000)  
Exercise Price, Cancelled (in dollars per share) $ 51.55 $ 52.05 $ 45.08  
Proceeds on Exercise, Cancelled $ (1,559) $ (598) $ (540)  
Number of Shares (in shares) 1,028,500 1,278,250 981,750 1,117,000
Exercise Price (in dollars per share) $ 33.51 $ 30.57 $ 24.05 $ 21.75
Minimum [Member]        
Exercise Price (in dollars per share) 11.90 11.90  
Exercise Price, Granted (in dollars per share) 26.42 26.37 36.54  
Exercise Price, Exercised (in dollars per share) 11.90 11.90 11.90  
Exercise Price, Cancelled (in dollars per share) 41.86 41.86 41.86  
Exercise Price (in dollars per share)   11.90 11.90
Maximum [Member]        
Exercise Price (in dollars per share) 53.02 46.44  
Exercise Price, Granted (in dollars per share) 34.09 53.02 50.25  
Exercise Price, Exercised (in dollars per share) 28.13 23.34 45.46  
Exercise Price, Cancelled (in dollars per share) $ 53.02 53.02 45.72  
Exercise Price (in dollars per share)   $ 53.02 $ 46.44