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Components of Net Periodic Benefits Costs (Detail) - Deferred Compensation Plan - USD ($)
$ in Thousands
12 Months Ended
Apr. 30, 2018
Apr. 30, 2017
Apr. 30, 2016
Defined Benefit Plan Disclosure [Line Items]      
Service cost $ 11,691 $ 5,507  
Interest cost 3,469 3,820 $ 3,423
Amortization of actuarial loss 2,308 3,051 2,924
Expected return on plan assets (1,594) (1,559) (682)
Net periodic benefit cost $ 15,874 $ 10,819 $ 5,665