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Comprehensive Income - Changes in Each Component of Accumulated Other Comprehensive (Loss) Income (Parenthetical) (Detail) - USD ($)
$ in Millions
12 Months Ended
Apr. 30, 2018
Apr. 30, 2017
Apr. 30, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Tax effect on reclassifications of realized net losses $ 0.8 $ 1.2 $ 1.1
Accumulated Defined Benefit Plan Adjustment      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Tax effects on unrealized gains (losses) 2.5 1.9 $ (2.3)
Unrealized (Losses) Gains on Interest Rate Swap      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Tax effects on unrealized gains (losses) 0.8 $ (0.4)  
Tax effect on reclassifications of realized net losses $ 0.3