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Components of Net Periodic Benefit Costs (Detail) - Deferred Compensation Plan - USD ($)
$ in Thousands
3 Months Ended
Jul. 31, 2016
Jul. 31, 2015
Defined Benefit Plan Disclosure [Line Items]    
Interest cost $ 1,062 $ 703
Amortization of actuarial loss 763 731
Service cost 609  
Expected return on plan assets (390)  
Net periodic benefit costs $ 2,044 $ 1,434