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Comprehensive Income - Changes in Each Component of Accumulated Other Comprehensive Loss (Parenthetical) (Detail) - USD ($)
$ in Millions
12 Months Ended
Apr. 30, 2021
Apr. 30, 2020
Apr. 30, 2019
Deferred Compensation and Pension Plan      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Tax effect on unrealized gains (losses) $ 1.1 $ (3.1) $ (2.3)
Tax effect on reclassification of realized net losses $ 1.0 0.8 0.4
Unrealized Gains on Interest Rate Swap      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Tax effect on unrealized gains (losses)   (0.2) (0.3)
Tax effect on reclassification of realized net (losses) gains   $ (0.1) $ 0.1