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Fair Value Measurement - Additional Information (Detail) - USD ($)
Jul. 31, 2015
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 Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash equivalents $ 55,400,000 $ 42,200,000
(Level 2) | Interest Rate Swap    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Fair value of the interest rate swap $ 181,000