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

The amount of the liability included in the deferred compensation plans that is unvested and therefore the participants have not earned the right to.

No definition available.

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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=116654751&loc=SL75117546-209714

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Amount of the liabilities for deferred compensation arrangements classified as other.

+ References

No definition available.

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