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Financial Instruments - Financial Instruments and Balance Sheet Classifications (Parenthetical) (Detail) - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Jul. 31, 2021 | Jul. 31, 2020 | Apr. 30, 2021
Investments Debt And Equity Securities [Abstract] |  |  |
Obligations for which assets are held in trust | $ 171.2 |  | $ 157.3
Gain on marketable securities | 5.2 | $ 11.5 |
Unvested obligations under deferred compensation plans | $ 24.7 |  | $ 26.5

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