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Retirement Benefits - Summary of Plan Assets by Asset Categories (Detail) - USD ($)
$ in Thousands
Apr. 30, 2019
Apr. 30, 2018
Apr. 30, 2017
Defined Benefit Plan Disclosure [Line Items]      
Defined benefit plan, Plan asset $ 19,035 $ 18,540 $ 17,198
Defined benefit plan, Plan asset percentage 100.00% 100.00%  
Equity Securities      
Defined Benefit Plan Disclosure [Line Items]      
Defined benefit plan, Plan asset $ 14,085 $ 9,643  
Defined benefit plan, Plan asset percentage 74.00% 52.00%  
Fixed Income      
Defined Benefit Plan Disclosure [Line Items]      
Defined benefit plan, Plan asset $ 4,754 $ 4,599  
Defined benefit plan, Plan asset percentage 25.00% 25.00%  
Cash and Cash Equivalents      
Defined Benefit Plan Disclosure [Line Items]      
Defined benefit plan, Plan asset $ 196 $ 4,298  
Defined benefit plan, Plan asset percentage 1.00% 23.00%