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Components of Net Periodic Benefit Costs (Detail) - Deferred Compensation Plan - USD ($)
$ in Thousands
12 Months Ended
Apr. 30, 2015
Apr. 30, 2014
Apr. 30, 2013
Defined Benefit Plan Disclosure [Line Items]      
Interest cost $ 2,835 $ 2,566 $ 2,868
Amortization of actuarial loss 3,029 3,111 2,357
Net periodic benefit cost $ 5,864 $ 5,677 $ 5,225