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Financial Instruments - Financial Instruments and Balance Sheet Classifications (Parenthetical) (Detail) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Oct. 31, 2021 | Oct. 31, 2020 | Oct. 31, 2021 | Oct. 31, 2020 | Apr. 30, 2021
Investments Debt And Equity Securities [Abstract] |  |  |  |  |
Obligations for which assets are held in trust | $ 174.6 |  | $ 174.6 |  | $ 157.3
Gain on marketable securities | 4.8 | $ 0.6 | 10.0 | $ 12.0 |
Unvested obligations under deferred compensation plans | $ 26.7 |  | $ 26.7 |  | $ 26.5

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