Note K - Stock-based Compensation Plans - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Apr. 30, 2019	Apr. 30, 2018	Apr. 30, 2017	Apr. 30, 2016
Number of Shares (in shares)	695,500	1,028,500	1,278,250
Proceeds on Exercise	$ 26,087	$ 26,683	$ 34,084	$ 39,081
Weighted Average Exercise Price (in dollars per share)	$ 30.50	$ 33.51	$ 30.57
Number of Shares, Granted (in shares)	145,000	25,000	45,000
Exercise Price, Granted (in dollars per share)	$ 54.58	$ 37.30	$ 28.90
Proceeds on Exercise, Granted	$ 7,915	$ 933	$ 1,301
Number of Shares, Exercised (in shares)	(275,000)	(323,000)	(264,500)
Exercise Price, Exercised (in dollars per share)	$ 30.95	$ 20.72	$ 17.92
Proceeds on Exercise, Exercised	$ (8,511)	$ (6,692)	$ (4,739)
Number of Shares, Cancelled (in shares)		(20,000)	(30,250)
Exercise Price, Cancelled (in dollars per share)		$ 46.61	$ 51.55
Proceeds on Exercise, Cancelled		$ (932)	$ (1,559)
Number of Shares, Expired (in shares)		(15,000)
Exercise Price, Expired (in dollars per share)		$ 47.26
Proceeds on Exercise, Expired		$ (710)
Number of Shares (in shares)	565,500	695,500	1,028,500	1,278,250
Weighted Average Exercise Price (in dollars per share)	$ 46.13	$ 30.50	$ 33.51	$ 30.57
Minimum [Member]
Exercise Price, Granted (in dollars per share)	53.30		26.42
Exercise Price, Exercised (in dollars per share)	18.86	11.90	11.90
Exercise Price, Cancelled (in dollars per share)		41.86	41.86
Exercise Price, Expired (in dollars per share)		44.52
Maximum [Member]
Exercise Price, Granted (in dollars per share)	54.85		34.09
Exercise Price, Exercised (in dollars per share)	$ 53.30	37.94	28.13
Exercise Price, Cancelled (in dollars per share)		53.02	$ 53.02
Exercise Price, Expired (in dollars per share)		$ 51.81