Employee Stock Plans - Components of Stock-Based Compensation Expense Recognized (Detail) - USD ($) $ in Thousands	3 Months Ended
	Jul. 31, 2018	Jul. 31, 2017
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense, pre-tax	$ 5,714	$ 4,696
Tax benefit from stock-based compensation expense	(1,682)	(1,378)
Total stock-based compensation expense, net of tax	4,032	3,318
Restricted Stock
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense, pre-tax	5,369	4,405
ESPP
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense, pre-tax	$ 345	$ 291