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Capital Stock and Changes in Capital Accounts (Details)
 12 Months Ended
Apr. 30, 2019
Vote
$ / shares
shares
Apr. 30, 2018
$ / shares
shares
Apr. 30, 2017
$ / shares
shares
2017 Share Repurchase Program [Member]      
Capital Stock [Abstract]      
Additional shares of common stock approved for repurchase under the share repurchase program (in shares)     4,000,000
Average price of shares repurchased during the period (in dollars per share) | $ / shares   $ 55.65 $ 52.80
Remaining number of shares authorized to be repurchased under the share repurchase program (in shares) 1,888,975    
Changes in Common Stock in Treasury [Abstract]      
Purchase of treasury shares (in shares)   713,177 953,188
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    
Changes in Common Stock [Abstract]      
Number of shares, beginning of year 70,110,603 70,086,000 69,798,000
Common stock class conversions and other 16,000 25,000 288,000
Number of shares issued, end of year (in shares) 70,126,963 70,110,603 70,086,000
Changes in Common Stock in Treasury [Abstract]      
Number of shares held, beginning of year (in shares) 21,853,257 22,097,000 21,709,000
Purchase of treasury shares (in shares) 1,192,000 713,000 953,000
Stock grants of fully vested Class A shares - common stock (in shares) 0 (20,000) (24,000)
Restricted shares, forfeited (in shares) 9,000 15,000 8,000
Restricted shares issued from exercise of stock options (in shares) (229,000) (788,000) (469,000)
Shares withheld for taxes (in shares) 130,000 116,000 97,000
Other (in shares) (1,000) (1,000) 83,000
Number of shares held, end of year (in shares) 22,633,869 21,853,257 22,097,000
Number of Common Stock outstanding, end of year (in shares) 47,493,000 48,258,000 47,989,000
Class A [Member] | Non-Performance Shares [Member]      
Changes in Common Stock in Treasury [Abstract]      
Restricted shares issued under stock-based compensation plans (in shares) (210,000) (153,000) (74,000)
Class A [Member] | Performance Share Units [Member]      
Changes in Common Stock in Treasury [Abstract]      
Restricted shares issued under stock-based compensation plans (in shares) (110,000) (126,000) (186,000)
Class A [Member] | 2017 Share Repurchase Program [Member]      
Capital Stock [Abstract]      
Average price of shares repurchased during the period (in dollars per share) | $ / shares $ 50.35    
Changes in Common Stock in Treasury [Abstract]      
Purchase of treasury shares (in shares) 1,191,496   952,667
Class B [Member]      
Common Stock [Abstract]      
Number of votes to which each share of common stock is entitled | Vote 1    
Changes in Common Stock [Abstract]      
Number of shares, beginning of year 13,071,067 13,096,000 13,392,000
Common stock class conversions and other (16,000) (25,000) (296,000)
Number of shares issued, end of year (in shares) 13,054,707 13,071,067 13,096,000
Changes in Common Stock in Treasury [Abstract]      
Number of shares held, beginning of year (in shares) 3,917,574 3,918,000 3,917,000
Purchase of treasury shares (in shares) 0 0 1,000
Number of shares held, end of year (in shares) 3,917,574 3,917,574 3,918,000
Number of Common Stock outstanding, end of year (in shares) 9,137,000 9,153,000 9,178,000
Class B [Member] | 2017 Share Repurchase Program [Member]      
Changes in Common Stock in Treasury [Abstract]      
Purchase of treasury shares (in shares)     521