Annual report pursuant to Section 13 and 15(d)

Comprehensive Income - Changes in Each Component of Accumulated Other Comprehensive Loss (Parenthetical) (Detail)

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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, 2022
Apr. 30, 2021
Apr. 30, 2020
Deferred Compensation and Pension Plan      
Accumulated Other Comprehensive Income Loss [Line Items]      
Tax effect on unrealized gains (losses) $ 6.0 $ 1.1 $ (3.1)
Tax effect on reclassification of realized net losses 0.5 $ 1.0 0.8
Unrealized Gains (Losses) on Marketable Securities      
Accumulated Other Comprehensive Income Loss [Line Items]      
Tax effect on unrealized gains (losses) $ (0.1)    
Unrealized Gains on Interest Rate Swap      
Accumulated Other Comprehensive Income Loss [Line Items]      
Tax effect on unrealized gains (losses)     (0.2)
Tax effect on reclassification of realized net losses     $ 0.1