Deferred Compensation and Retirement Plans - Components of Net Periodic Benefits Costs (Detail) - Deferred Compensation Plan - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Jan. 31, 2018	Jan. 31, 2017	Jan. 31, 2018	Jan. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Service cost		$ 3,235	$ 1,793	$ 8,526	$ 3,981
Interest cost		958	1,062	2,876	3,185
Amortization of actuarial loss		577	763	1,731	2,289
Expected return on plan assets	[1]	(398)	(389)	(1,195)	(1,169)
Net periodic benefit costs		$ 4,372	$ 3,229	$ 11,938	$ 8,286

[1] The expected long-term rate of return on plan assets is 6.50% for January 31, 2018 and 2017.