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Financial Instruments - Financial Instruments and Balance Sheet Classification (Parenthetical) (Detail) - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Jul. 31, 2020 | Jul. 31, 2019 | Apr. 30, 2020
Investments Debt And Equity Securities [Abstract] |  |  |
Obligations for which assets are held in trust | $ 137.4 |  | $ 124.6
Gain on marketable securities | 11.5 | $ 1.9 |
Unvested obligations under deferred compensation plans | $ 24.1 |  | $ 21.7

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