Retirement Benefits - Summary of Components of Net Periodic Pension Cost (Detail) - USD ($) $ in Thousands	12 Months Ended
	Apr. 30, 2018	Apr. 30, 2017	Apr. 30, 2016
Retirement Benefits [Abstract]
Interest cost	$ 875	$ 927	$ 912
Expected return on plan assets	(1,314)	(1,244)	(1,363)
Recognition of net loss	1,133	1,257	1,223
Net periodic pension cost	$ 694	$ 940	$ 772