Financial Instruments - Financial Instruments and Balance Sheet Classification (Parenthetical) (Detail) - USD ($) $ in Millions	12 Months Ended
Apr. 30, 2021	Apr. 30, 2020	Apr. 30, 2019
Investments Debt And Equity Securities [Abstract]
Obligations for which assets are held in trust	$ 157.3	$ 124.6
Gain (loss) on marketable securities	38.5	(1.8)	$ 8.1
Unvested obligations under deferred compensation plans	$ 26.5	$ 21.7

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).

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=120269885&loc=SL75117546-209714

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