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Retirement Benefits - Summary of Components of Net Periodic Pension Cost (Detail) - USD ($)
$ in Thousands
12 Months Ended
Apr. 30, 2017
Apr. 30, 2016
Apr. 30, 2015
Compensation and Retirement Disclosure [Abstract]      
Interest cost $ 927 $ 912 $ 893
Expected return on plan assets (1,244) (1,363) (1,324)
Recognition of net loss 1,257 1,223 934
Net periodic pension cost $ 940 $ 772 $ 503