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Common Shares (Details)
$ / shares in Units, $ in Millions
 12 Months Ended
Apr. 30, 2018
votes_per_share
right
$ / shares
shares
Apr. 30, 2017
USD ($)
shares
Apr. 30, 2016
shares
Feb. 22, 2017
shares
Equity, Class of Treasury Stock [Line Items]        
Number of votes each holder of a common share outstanding is entitled | votes_per_share 1      
Number of votes per share | votes_per_share 10      
Number of votes per share after change in beneficial ownership | votes_per_share 1      
Number of years required to pass after change in beneficial ownership 4 years      
Number of share purchase rights | right 1      
Minimum percentage of outstanding common shares held by persons or group (or more) 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 | $ / shares $ 0.001      
Shares repurchased during period, shares 0   0  
Shares remaining for repurchase 3,600,000      
10b5-1 Trading Plan [Member]        
Equity, Class of Treasury Stock [Line Items]        
Stock repurchase program, number of shares authorized       3,000,000.0
Shares repurchased during period, shares   3,000,000    
Stock repurchased during period | $   $ 418.1