Benefit Plans (Details) $ in Thousands	12 Months Ended
	Apr. 30, 2020 USD ($) executive shares	Apr. 30, 2019 USD ($) shares	Apr. 30, 2018 USD ($)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Accrued deferred compensation liability	$ 3,793	$ 2,870
Expected future payments, 2021	1,511
Expected future payments, 2022	401
Expected future payments, 2023	401
Expected future payments, 2024	401
Expected future payments, 2025	401
Deferred compensation expense incurred	2,727	(97)	$ 131
Other Postretirement Benefits Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Employer discretionary contribution	$ 10,571	$ 9,918	9,614
Common stock held by trustee of the 401K plan (shares) | shares	1,113,882	1,261,258
Supplemental Employee Retirement Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Number of executives covered under SERP | executive	2
Annual benefit amount (percent of base compensation)	50.00%
Duration of benefits	20 years
Accrued deferred compensation liability	$ 3,434	$ 3,800
Pension and post-retirement expense incurred	269	$ 221	$ 112
Expected future payments, 2021	635
Expected future payments, 2022	635
Expected future payments, 2023	635
Expected future payments, 2024	354
Expected future payments, 2025	$ 354
Minimum | Supplemental Employee Retirement Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Discount rate	2.04%	3.78%
Maximum | Supplemental Employee Retirement Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Discount rate	2.44%	4.01%