Deferred Compensation and Retirement Plans - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jan. 31, 2018	Jan. 31, 2017	Jan. 31, 2018	Jan. 31, 2017	Apr. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Increase in market value of the underlying COLI investments			$ 6,020	$ 3,316
Recognized investment income(expense)	$ 7,200	$ 3,900	14,022	7,090
CSV of COLI Contracts
Defined Benefit Plan Disclosure [Line Items]
Gross CSV	185,000		185,000		$ 180,300
Outstanding policy loans	66,800		66,800		$ 67,200
Increase in market value of the underlying COLI investments	1,800	100	6,000	3,300
Executive Capital Accumulation Plan
Defined Benefit Plan Disclosure [Line Items]
Gain (loss) on deferred compensation plan	$ 7,200	$ 4,100	$ 14,400	$ 6,700
Executive Capital Accumulation Plan | Minimum
Defined Benefit Plan Disclosure [Line Items]
Deferred compensation arrangement vesting period			4 years
Executive Capital Accumulation Plan | Maximum
Defined Benefit Plan Disclosure [Line Items]
Deferred compensation arrangement vesting period			5 years