Deferred Compensation and Retirement Plans - Components of Net Periodic Benefits Costs (Detail) - Deferred Compensation Plan - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Oct. 31, 2017	Oct. 31, 2016	Oct. 31, 2017	Oct. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Service cost		$ 3,165	$ 1,579	$ 5,291	$ 2,188
Interest cost		959	1,061	1,918	2,123
Amortization of actuarial loss		577	763	1,154	1,526
Expected return on plan assets	[1]	(398)	(390)	(797)	(780)
Net periodic benefit costs		$ 4,303	$ 3,013	$ 7,566	$ 5,057

[1] The expected long-term rate of return on plan assets is 6.50% for October 31, 2017 and 2016.