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Components of Stock-Based Compensation Expense Recognized (Detail) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Jul. 31, 2013
Jul. 31, 2012
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]    
Total stock-based compensation expense, pre-tax $ 2,960 $ 3,196
Tax benefit from stock-based compensation expense (1,192) (1,164)
Total stock-based compensation expense, net of tax 1,768 2,032
Restricted Stock
    
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]    
Total stock-based compensation expense, pre-tax 2,820 2,986
Stock Options
    
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]    
Total stock-based compensation expense, pre-tax $ 140 $ 210