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Components of Net Periodic Benefits Costs (Detail) - Deferred Compensation Plan - USD ($)
$ in Thousands
 3 Months Ended
Jul. 31, 2017
Jul. 31, 2016
Defined Benefit Plan Disclosure [Line Items]    
Service cost $ 2,126 $ 609
Interest cost 959 1,062
Amortization of actuarial loss 577 763
Expected return on plan assets [1] (399) (390)
Net periodic benefit costs $ 3,263 $ 2,044
[1] The expected long-term rate of return on plan assets is 6.50% for July 31,2017 and 2016.