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Components of Net Periodic Benefit Costs (Detail) (Deferred Compensation Plan, USD $)
In Thousands, unless otherwise specified
3 Months Ended 6 Months Ended
Oct. 31, 2014
Oct. 31, 2013
Oct. 31, 2014
Oct. 31, 2013
Deferred Compensation Plan
       
Defined Benefit Plan Disclosure [Line Items]        
Amortization of actuarial loss $ 762 $ 779 $ 1,525 $ 1,559
Interest cost 748 676 1,495 1,352
Net periodic benefit costs $ 1,510 $ 1,455 $ 3,020 $ 2,911