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Fair Value Measurement - Additional Information (Detail) - USD ($)
$ in Millions
Jan. 31, 2016
Apr. 30, 2015
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Fair value of the interest rate swap liability $ 1.2  
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 $ 105.2 $ 42.2
(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 liability $ 1.2