Capital Stock and Changes in Capital Accounts (Details) $ / shares in Units, $ in Millions	12 Months Ended
	Apr. 30, 2020 USD ($) Vote $ / shares shares	Apr. 30, 2019 $ / shares shares	Apr. 30, 2018 $ / shares shares	Apr. 30, 2017 shares
Share repurchases [Abstract]
Average Price (in dollars per share) | $ / shares	$ 43.05	$ 50.35	$ 55.65
2017 Share Repurchase Program [Member]
Capital Stock [Abstract]
Additional shares/dollars of common stock approved for repurchase under the share repurchase program (in shares in 2019 and dollars in 2020)				4,000,000
Remaining number of shares/dollars authorized to be repurchased under the share repurchase program in 2017 program (shares) and 2020 program (dollars)	806,758
2020 Share Repurchase Program [Member]
Capital Stock [Abstract]
Amount of additional stock authorized to be repurchased | $	$ 200
Remaining amount authorized to be repurchased under the share repurchase program | $	$ 200
Share repurchases [Abstract]
Shares Repurchased (in shares)	0
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
Share repurchases [Abstract]
Shares Repurchased (in shares)	1,079,936	1,191,496	713,177
Changes in Common Stock [Abstract]
Number of shares, beginning of year (in shares)	70,127,000	70,111,000	70,086,000
Common stock class conversions and other	39,000	16,000	25,000
Number of shares issued, end of period (in shares)	70,166,000	70,127,000	70,111,000	70,086,000
Changes in Common Stock in Treasury [Abstract]
Number of shares held, beginning of year (in shares)	22,634,000	21,853,000	22,097,000
Purchase of treasury shares (in shares)	1,080,000	1,192,000	713,000
Shares issued under the Director Plan to Directors	(97,000)	(5,000)	(20,000)
Stock grants of fully vested Class A shares - common stock (in shares)	0	0	(20,000)
Restricted shares, forfeited (in shares)	1,000	9,000	15,000
Restricted shares issued from exercise of stock options (in shares)	(34,000)	(229,000)	(788,000)
Shares withheld for taxes (in shares)	122,000	130,000	116,000
Other (in shares)	(1,000)	(1,000)	(1,000)
Number of shares held, end of period (in shares)	23,405,000	22,634,000	21,853,000	22,097,000
Number of Common Stock outstanding, end of period (in shares)	46,761,000	47,493,000	48,258,000
Class A [Member] | Non-Performance Shares [Member]
Changes in Common Stock in Treasury [Abstract]
Restricted shares issued under stock-based compensation plans (in shares)	(232,000)	(205,000)	(133,000)
Class A [Member] | Performance Share Units [Member]
Changes in Common Stock in Treasury [Abstract]
Restricted shares issued under stock-based compensation plans (in shares)	(68,000)	(110,000)	(126,000)
Class B [Member]
Common Stock [Abstract]
Number of votes to which each share of common stock is entitled | Vote	1
Share repurchases [Abstract]
Shares Repurchased (in shares)	2,281	0	0
Changes in Common Stock [Abstract]
Number of shares, beginning of year (in shares)	13,055,000	13,071,000	13,096,000
Common stock class conversions and other	(39,000)	(16,000)	(25,000)
Number of shares issued, end of period (in shares)	13,016,000	13,055,000	13,071,000	13,096,000
Changes in Common Stock in Treasury [Abstract]
Number of shares held, beginning of year (in shares)	3,918,000	3,918,000	3,918,000
Purchase of treasury shares (in shares)	2,000	0	0
Number of shares held, end of period (in shares)	3,920,000	3,918,000	3,918,000	3,918,000
Number of Common Stock outstanding, end of period (in shares)	9,096,000	9,137,000	9,153,000