Deferred Compensation and Retirement Plans - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Jul. 31, 2018	Jul. 31, 2017	Apr. 30, 2018
Defined Benefit Plan Disclosure [Line Items]
Increase in market value of the underlying COLI investments	$ 1,347	$ 2,485
Recognized investment income(expense)	4,001	3,429
Executive Capital Accumulation Plan
Defined Benefit Plan Disclosure [Line Items]
Gain (loss) on deferred compensation plan	$ 4,200	3,700
Minimum
Defined Benefit Plan Disclosure [Line Items]
Option to receive employee benefits by quarterly installments periods	1 year
Minimum | Executive Capital Accumulation Plan
Defined Benefit Plan Disclosure [Line Items]
Deferred compensation arrangement vesting period	4 years
Maximum
Defined Benefit Plan Disclosure [Line Items]
Option to receive employee benefits by quarterly installments periods	15 years
Maximum | Executive Capital Accumulation Plan
Defined Benefit Plan Disclosure [Line Items]
Deferred compensation arrangement vesting period	5 years
CSV of COLI Contracts
Defined Benefit Plan Disclosure [Line Items]
Gross CSV	$ 188,500		$ 186,800
Outstanding policy loans	66,700		$ 66,700
Increase in market value of the underlying COLI investments	$ 1,300	$ 2,500