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Note K - Stock Option Activity (Details) - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Apr. 30, 2016
Apr. 30, 2015
Apr. 30, 2014
Apr. 30, 2013
Minimum [Member]        
Exercise Price (in dollars per share) $ 11.90 $ 11.90 $ 11.90  
Exercise Price, Granted (in dollars per share) 26.37 36.54 44.52  
Exercise Price, Exercised (in dollars per share) 11.90 11.90 23.34  
Exercise Price (in dollars per share)   11.90 11.90 $ 11.90
Exercise Price, Cancelled (in dollars per share) 41.86 41.86    
Maximum [Member]        
Exercise Price (in dollars per share) 53.02 46.44 45.72  
Exercise Price, Granted (in dollars per share) 53.02 50.25 46.44  
Exercise Price, Exercised (in dollars per share) 23.34 45.46 23.75  
Exercise Price (in dollars per share)   53.02 $ 46.44 $ 45.72
Exercise Price, Cancelled (in dollars per share) $ 53.02 $ 45.72    
Number of Shares (in shares) 981,750 1,117,000 1,122,500  
Exercise Price (in dollars per share) $ 24.05 $ 21.75 $ 21.28  
Proceeds on Exercise $ 39,081 $ 23,608 $ 24,294 $ 23,892
Number of Shares, Granted (in shares) 338,750 89,000 25,000  
Exercise Price, Granted (in dollars per share) $ 48.62 $ 44.91 $ 44.90  
Proceeds on Exercise, Granted $ 16,471 $ 3,997 $ 1,122  
Number of Shares, Exercised (in shares) (30,750) (212,250) (30,500)  
Exercise Price, Exercised (in dollars per share) $ 13 $ 19.52 $ 23.58  
Proceeds on Exercise, Exercised $ (400) $ (4,143) $ (720)  
Number of Shares (in shares) 1,278,250 981,750 1,117,000 1,122,500
Exercise Price (in dollars per share) $ 30.57 $ 24.05 $ 21.75 $ 21.28
Number of Shares, Cancelled (in shares) (11,500) (12,000)    
Exercise Price, Cancelled (in dollars per share) $ 52.05 $ 45.08    
Proceeds on Exercise, Cancelled $ (598) $ (540)