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Financial Instruments - Additional Information (Detail) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Oct. 31, 2016
Oct. 31, 2015
Oct. 31, 2016
Oct. 31, 2015
Apr. 30, 2016
Financial Instrument [Line Items]          
Proceeds from maturities of available-for-sale securities   $ 8.1   $ 11.1  
Not Designated as Hedge Instrument | Foreign Exchange Forward Contracts          
Financial Instrument [Line Items]          
Foreign currency gain $ 0.2   $ 0.2    
Not Designated as Hedge Instrument | Foreign Exchange Forward Contracts | Other Accrued Liabilities | Derivatives Purchased          
Financial Instrument [Line Items]          
Derivative notional amount 2.9   2.9   $ 14.5
Not Designated as Hedge Instrument | Foreign Exchange Forward Contracts | Other Accrued Liabilities | Derivatives Sold          
Financial Instrument [Line Items]          
Derivative notional amount $ 73.0   $ 73.0   $ 44.3