Retirement Benefits - Summary of Change in Projected Benefit Obligations and Change in Fair Value of Plan Assets (Detail) - USD ($) $ in Thousands	12 Months Ended
	Apr. 30, 2018	Apr. 30, 2017	Apr. 30, 2016
Retirement Benefits [Abstract]
Accumulated Benefit Obligation, April 30	$ 21,544	$ 21,313
Change in Projected Benefit Obligations
Projected benefit obligations, beginning of year	21,313	21,156
Interest cost	875	927	$ 912
Actuarial loss	480	299
Actual benefits paid	(1,124)	(1,069)
Projected benefit obligations, end of year	21,544	21,313	21,156
Change in Plan Assets
Fair value of plan assets, beginning of year	17,198	15,817
Actual return on plan assets	1,866	1,895
Employer contributions	600	555
Actual benefits paid	(1,124)	(1,069)
Fair value of plan assets, end of year	18,540	17,198	$ 15,817
Funded status-under	(3,004)	(4,115)
Amounts Recognized in the Consolidated Balance Sheets consist of:
Noncurrent liabilities	(3,004)	(4,115)
Amounts recognized in accumulated other comprehensive income (loss) consist of:
Net actual loss	7,481	8,686
Deferred tax benefit	(1,825)	(3,223)
After-tax actuarial loss	$ 5,656	$ 5,463
Weighted-Average Assumptions Used to Determine Benefit Obligations at April 30
Discount rate	4.10%	4.10%
Rate of compensation increase	0.00%	0.00%
Mortality table	Apr 30, 2018 RP-2014 and Apr 30,2017 RP-2014
Projection scale	Apr 30, 2018 MP-2017 and Apr 30,2017 MP-2016
Weighted-Average Assumptions Used to Determine Net Periodic Benefit Cost for Years Ended April 30
Discount rate	4.10%	4.50%
Expected long-term return on plan assets	7.75%	8.00%
Rate of compensation increase	0.00%	0.00%