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Components of Net Periodic Benefit Costs (Detail) - Deferred Compensation Plan - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Oct. 31, 2016
Oct. 31, 2015
Oct. 31, 2016
Oct. 31, 2015
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 1,579   $ 2,188  
Interest cost 1,061 $ 703 2,123 $ 1,406
Amortization of actuarial loss 763 731 1,526 1,462
Expected return on plan assets (390)   (780)  
Net periodic benefit costs $ 3,013 $ 1,434 $ 5,057 $ 2,868