Components of Net Periodic Benefit Costs (Detail) (Deferred Compensation Plan, USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Jan. 31, 2013	Jan. 31, 2012	Jan. 31, 2013	Jan. 31, 2012
Deferred Compensation Plan
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 756	$ 884	$ 2,268	$ 2,652
Amortization of actuarial loss	594	355	1,782	1,065
Net periodic benefit costs	$ 1,350	$ 1,239	$ 4,050	$ 3,717