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Retirement Benefits - Summary of Components of Net Periodic Pension Cost (Detail) - USD ($)
$ in Thousands
12 Months Ended
Apr. 30, 2015
Apr. 30, 2014
Apr. 30, 2013
Compensation and Retirement Disclosure [Abstract]      
Interest cost $ 893 $ 857 $ 906
Expected return on plan assets (1,324) (1,282) (1,213)
Recognition of net loss 934 1,142 1,102
Net periodic pension cost $ 503 $ 717 $ 795