Benefit Plans (Details) $ in Thousands	12 Months Ended
	Apr. 30, 2017 USD ($) employee executive shares	Apr. 30, 2016 USD ($) shares	Apr. 30, 2015 USD ($)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Number of individuals with deferred compensation agreements | employee	3
Accrued deferred compensation liability	$ 3,825	$ 4,043
Expected future payments, 2018	507
Expected future payments, 2019	457
Expected future payments, 2020	432
Expected future payments, 2021	432
Expected future payments, 2022	432
Deferred compensation expense incurred	370	238	$ 219
Other Postretirement Benefits Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Employer discretionary contribution	$ 8,181	$ 6,560	5,852
Common stock held by trustee of the 401K plan (shares) | shares	1,401,764	1,419,841
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	$ 4,737	$ 5,230
Discount rate	4.00%	3.80%
Pension and post-retirement expense incurred	$ 131	$ 230	$ 326
Expected future payments, 2018	625
Expected future payments, 2019	625
Expected future payments, 2020	625
Expected future payments, 2021	625
Expected future payments, 2022	$ 625