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Components of Net Periodic Benefit Costs (Detail) (Deferred Compensation Plan, USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Oct. 31, 2012	Oct. 31, 2011	Oct. 31, 2012	Oct. 31, 2011
Deferred Compensation Plan
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 756	$ 884	$ 1,512	$ 1,768
Amortization of actuarial loss	594	355	1,188	710
Net periodic benefit costs	$ 1,350	$ 1,239	$ 2,700	$ 2,478

- Definition

The increase in a defined benefit pension plan's projected benefit obligation or a defined benefit postretirement plan's accumulated postretirement benefit obligation due to the passage of time.

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