Benefit Plans (Details) $ in Thousands	12 Months Ended
	Apr. 30, 2015 USD ($) individual executive shares	Apr. 30, 2014 USD ($) shares	Apr. 30, 2013 USD ($)
401(K) Plan
Defined Contribution Plan Disclosure
Employer discretionary contribution	$ 5,852	$ 5,348	$ 4,949
Common stock held by trustee of the 401K plan | shares	1,453,251	1,488,510
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 (in years)	20 years
Accrued deferred compensation liability	$ 5,626	$ 5,925
Discount rate	4.00%	4.40%
Pension and post-retirement expense incurred	$ 326	$ 93	471
2016	625
2017	625
2018	625
2019	625
2020	625
Other Postretirement Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
2016	167
2017	497
2018	497
2019	447
2020	$ 422
Number of individuals with deferred compensation agreements | individual	3
Number of current employees with deferred compensation agreements | individual	1
Number of former employees with deferred compensation agreements | individual	2
Accrued deferred compensation liability	$ 3,960	3,950
Deferred compensation expense incurred	$ 219	$ 156	$ 438