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Changes in Each Component of Accumulated Other Comprehensive Income (Loss) (Parenthetical) (Detail) - USD ($)
$ in Millions
3 Months Ended
Jul. 31, 2017
Jul. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]    
Tax effect on reclassifications of realized net losses $ 0.2 $ 0.3
Tax effect on unrealized (losses) 0.1  
Unrealized (Losses) on Interest Rate Swap    
Accumulated Other Comprehensive Income (Loss) [Line Items]    
Tax effect on reclassifications of realized net losses $ 0.1