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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, 2013
Oct. 31, 2012
Oct. 31, 2013
Oct. 31, 2012
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]        
Total stock-based compensation expense, pre-tax $ 2,832 $ 3,073 $ 5,792 $ 6,269
Tax benefit from stock-based compensation expense (865) (1,581) (2,057) (2,745)
Total stock-based compensation expense, net of tax 1,967 1,492 3,735 3,524
Restricted Stock
        
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]        
Total stock-based compensation expense, pre-tax 2,727 2,847 5,547 5,833
Stock Options
        
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]        
Total stock-based compensation expense, pre-tax $ 105 $ 226 $ 245 $ 436