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Comprehensive Income - Changes in Each Component of Accumulated Other Comprehensive Income (Loss), Net (Parenthetical) (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Jan. 31, 2018
Jan. 31, 2017
Jan. 31, 2018
Jan. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Tax effect on reclassifications of realized net losses $ 0.2 $ 0.3 $ 0.7 $ 0.9
Unrealized (Losses) Gains on Interest Rate Swap        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Tax effect on unrealized gains 0.6   0.6  
Tax effect on reclassifications of realized net losses $ 0.1   $ 0.3