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Financial Instruments - Financial Instruments and Balance Sheet Classification (Parenthetical) (Detail) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Oct. 31, 2020 | Oct. 31, 2019 | Oct. 31, 2020 | Oct. 31, 2019 | Apr. 30, 2020
Investments Debt And Equity Securities [Abstract] |  |  |  |  |
Obligations for which assets are held in trust | $ 133.8 |  | $ 133.8 |  | $ 124.6
Gain on marketable securities | 0.6 | $ 1.2 | 12.0 | $ 3.1 |
Unvested obligations under deferred compensation plans | $ 24.4 |  | $ 24.4 |  | $ 21.7

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