Employee Stock Plans - Components of Stock-Based Compensation Expense Recognized (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Oct. 31, 2017	Oct. 31, 2016	Oct. 31, 2017	Oct. 31, 2016
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense, pre-tax	$ 5,587	$ 4,605	$ 10,283	$ 9,520
Tax benefit from stock-based compensation expense	(1,670)	(1,184)	(3,048)	(2,668)
Total stock-based compensation expense, net of tax	3,917	3,421	7,235	6,852
Restricted Stock
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense, pre-tax	5,309	4,352	9,714	9,091
ESPP
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense, pre-tax	$ 278	$ 253	$ 569	$ 429