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Fair Value Measurement - Additional Information (Detail) - USD ($)
Apr. 30, 2017
Apr. 30, 2016
Apr. 30, 2015
5.000% Senior Notes due 2018      
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]      
Debt instrument, interest rate 5.00%   5.00%
(Level 1) | Fair Value on Recurring Basis      
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]      
Cash and cash equivalents $ 61,500,000 $ 191,300,000  
Level 3      
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]      
Level 3 financial assets or liabilities   $ 0