Stock-Based Compensation Stock Option Plans (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Apr. 30, 2020	Apr. 30, 2019	Apr. 30, 2018
Shares
Options granted (in shares)	0	0	0
Stock options
Intrinsic value of options
Intrinsic value of options exercised	$ 833	$ 371	$ 2,407
Intrinsic value of options outstanding	11,779	15,569
Intrinsic value of exercisable options	$ 11,242	$ 13,950
Restated 2006 Plan | Stock options
Shares
Outstanding at the beginning of the year (in shares)	337,026	339,026	446,624
Options exercised (in shares)	(3,000)	(2,000)	(107,598)
Outstanding at the end of the year (in shares)	334,026	337,026	339,026
Options exercisable (in shares)	318,026
Weighted Average Exercise Price
Outstanding at the beginning of the year (in dollars per share)	$ 25.25	$ 25.29	$ 24.93
Options exercised (in dollars per share)	31.15	32.19	23.80
Outstanding at the end of the year (in dollars per share)	25.19	$ 25.25	$ 25.29
Options exercisable (in dollars per share)	$ 25.12
1992 Plan | Stock options
Shares
Outstanding at the beginning of the year (in shares)	14,302	18,302	43,415
Options exercised (in shares)	(13,189)	(4,000)	(25,113)
Outstanding at the end of the year (in shares)	1,113	14,302	18,302
Options exercisable (in shares)	1,113
Weighted Average Exercise Price
Outstanding at the beginning of the year (in dollars per share)	$ 0.59	$ 0.59	$ 0.59
Options exercised (in dollars per share)	0.59	0.59	0.59
Outstanding at the end of the year (in dollars per share)	0.59	$ 0.59	$ 0.59
Options exercisable (in dollars per share)	$ 0.59