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Capital Stock and Changes in Capital Accounts (Details)
 12 Months Ended
Apr. 30, 2018
Vote
$ / shares
shares
Apr. 30, 2017
$ / shares
shares
2017 Share Repurchase Program [Member]    
Equity, Class of Treasury Stock [Line Items]    
Additional shares of common stock approved for repurchase under the share repurchase program (in shares)   4,000,000
Number of shares repurchased during the period (in shares)   953,188
Average price of shares repurchased during the period (in dollars per share) | $ / shares   $ 52.80
Remaining number of shares authorized to be repurchased under the share repurchase program (in shares) 3,080,471  
Class A [Member]    
Common Stock [Abstract]    
Class A Common shares into which each share of Class B Common Stock is convertible (in shares) 1  
Percentage of the Board of Directors elected by Class A common stockholders 30.00%  
Number of votes to which each share of common stock is entitled | Vote 0.1  
Class A [Member] | 2017 Share Repurchase Program [Member]    
Equity, Class of Treasury Stock [Line Items]    
Number of shares repurchased during the period (in shares) 713,177 952,667
Average price of shares repurchased during the period (in dollars per share) | $ / shares $ 55.65  
Class B [Member]    
Common Stock [Abstract]    
Number of votes to which each share of common stock is entitled | Vote 1  
Class B [Member] | 2017 Share Repurchase Program [Member]    
Equity, Class of Treasury Stock [Line Items]    
Number of shares repurchased during the period (in shares)   521