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Components of Stock-Based Compensation Expense Recognized (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Jul. 31, 2012	Jul. 31, 2011
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense, pre-tax	$ 3,196	$ 3,170
Tax benefit from stock-based compensation expense	(1,164)	(1,146)
Total stock-based compensation expense, net of tax	2,032	2,024
Restricted Stock
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense, pre-tax	2,986	2,784
Stock Options And Stock Appreciation Rights
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense, pre-tax	210	258
Employee Stock Purchase Plan
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense, pre-tax		$ 128

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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