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Common Shares (Details)
 12 Months Ended
Apr. 30, 2015
USD ($)
Votes
Rights
yr
shares
Apr. 30, 2014
USD ($)
shares
Apr. 30, 2013
USD ($)
shares
Common shares (Additional Textual) [Abstract]      
Number of votes each holder of a common share outstanding is entitled | Votes 1    
Number of votes per share | Votes 10    
Number of years there has not been a change in beneficial ownership | yr 4    
Number of votes per share after change in beneficial ownership | Votes 1    
Number of years required to pass after change in beneficial ownership | yr 4    
Number of share purchase rights | Rights 1    
Minimum percentage of outstanding common shares held by persons or group 10.00%    
Number of shares shareholder can purchase at discounted price 1    
Number of common shares issued for each right as part of exchange option 1    
Value to redeem per right | $ $ 0.001    
Shares repurchased during period 0 4,900,000 4,000,000
Purchase amount for shares repurchased | $ $ 0 $ 495,000,000 $ 359,400,000
Shares remaining for repurchase 10,000,000