Retirement Benefits - Summary of Plan Assets by Asset Categories (Detail) - USD ($) $ in Thousands	Apr. 30, 2018	Apr. 30, 2017	Apr. 30, 2016
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, Plan asset	$ 18,540	$ 17,198	$ 15,817
Defined benefit plan, Plan asset percentage	100.00%	100.00%
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, Plan asset	$ 9,643	$ 10,611
Defined benefit plan, Plan asset percentage	52.00%	62.00%
Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, Plan asset	$ 4,599	$ 6,466
Defined benefit plan, Plan asset percentage	25.00%	37.00%
Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, Plan asset	$ 4,298	$ 121
Defined benefit plan, Plan asset percentage	23.00%	1.00%