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Preferred And Common Stock (Stock Option Activity) (Details) (USD $)
 12 Months Ended
Apr. 30, 2013
Apr. 30, 2012
Apr. 30, 2011
Preferred And Common Stock [Line Items]      
Number of Shares, Exercised (198,200) (159,600) (184,441)
Number of shares, outstanding 854,809    
Weighted Average Exercise Price, Outstanding $ 32.59 $ 23.38 $ 22.78
Weighted Average Exercise Price, Granted $ 0.00 $ 44.39 $ 0.00
Weighted Average Exercise Price, Exercised $ 23.82 $ 20.36 $ 20.24
Weighted Average Exercise Price, Forfeited $ 14.08 $ 36.19 $ 0.00
Weighted Average Exercise Price, Outstanding $ 34.64 $ 32.59 $ 23.38
Restricted Stock Units (RSUs) [Member]
      
Preferred And Common Stock [Line Items]      
Number of Shares, Outstanding 66,198 28,000   
Number of Shares, Granted 46,998 38,198 28,000
Number of Shares, Exercised         
Number of Shares, Forfeited         
Number of shares, outstanding 113,196 66,198 28,000
Stock Options [Member]
      
Preferred And Common Stock [Line Items]      
Number of Shares, Outstanding 1,053,509 775,109 959,550
Number of Shares, Granted   441,000  
Number of Shares, Exercised (198,200) (159,600) (184,441)
Number of Shares, Forfeited (500) (3,000)  
Number of shares, outstanding 854,809 1,053,509 775,109