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Property, Plant, and Equipment - Summary of Depreciation and Amortization Expense (Parenthetical) (Detail) - USD ($)
$ in Millions
12 Months Ended
Apr. 30, 2018
Apr. 30, 2017
Apr. 30, 2016
Property Plant And Equipment [Abstract]      
Amortization expenses recorded as a result of acquisition $ 20.8 $ 18.4 $ 9.9