Components of Net Periodic Benefit Costs (Detail) - Deferred Compensation Plan - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jan. 31, 2016	Jan. 31, 2015	Jan. 31, 2016	Jan. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Amortization of actuarial loss	$ 730	$ 763	$ 2,192	$ 2,288
Interest cost	703	747	2,109	2,242
Net periodic benefit costs	$ 1,433	$ 1,510	$ 4,301	$ 4,530