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Financial Instruments - Financial Instruments and Balance Sheet Classifications (Parenthetical) (Detail) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Jan. 31, 2022 | Jan. 31, 2021 | Jan. 31, 2022 | Jan. 31, 2021 | Apr. 30, 2021
Investments Debt And Equity Securities [Abstract] |  |  |  |  |
Obligations for which assets are held in trust | $ 175.2 |  | $ 175.2 |  | $ 157.3
Gain (loss) on marketable securities | (7.7) | $ 15.2 | 2.4 | $ 27.2 |
Unvested obligations under deferred compensation plans | $ 25.9 |  | $ 25.9 |  | $ 26.5

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- Definition Amount of unrealized and realized gain (loss) on investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 321 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=123583765&loc=SL75117546-209714 + Details
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