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Components of Net Periodic Benefit Costs (Detail) (Deferred Compensation Plan, USD $)
In Thousands, unless otherwise specified
3 Months Ended
Jul. 31, 2013
Jul. 31, 2012
Deferred Compensation Plan
   
Defined Benefit Plan Disclosure [Line Items]    
Amortization of actuarial loss $ 780 $ 594
Interest cost 676 756
Net periodic benefit costs $ 1,456 $ 1,350