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Financial Instruments - Financial Instruments and Balance Sheet Classification (Parenthetical) (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Apr. 30, 2020 | Apr. 30, 2019 | Apr. 30, 2018
Investment Holdings [Line Items] |  |  |
Obligations for which assets are held in trust | $ 124.6 | $ 122.3 |
Gain (loss) on marketable securities | (1.8) | 8.1 | $ 10.3
Unvested obligations under deferred compensation plans | 21.7 | 24.6 |
Mutual Funds |  |  |
Investment Holdings [Line Items] |  |  |
Gain (loss) on marketable securities | $ (1.8) | $ 8.1 | $ 10.3

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