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Capital Stock and Changes in Capital Accounts (Details)
 12 Months Ended
Apr. 30, 2017
Vote
$ / shares
shares
Equity, Class of Treasury Stock [Line Items]  
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) 3,793,648
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
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 B [Member]  
Common Stock [Abstract]  
Number of votes to which each share of common stock is entitled | Vote 1