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Components of Stock-Based Compensation Expense Recognized (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Oct. 31, 2014	Oct. 31, 2013	Oct. 31, 2014	Oct. 31, 2013
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense, pre-tax	$ 3,640	$ 2,832	$ 6,959	$ 5,792
Tax benefit from stock-based compensation expense	(1,114)	(865)	(2,096)	(2,057)
Total stock-based compensation expense, net of tax	2,526	1,967	4,863	3,735
Restricted Stock
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense, pre-tax	3,617	2,727	6,869	5,547
Stock Options
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense, pre-tax	$ 23	$ 105	$ 90	$ 245