Financial Instruments - Financial Instruments and Balance Sheet Classification (Parenthetical) (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Oct. 31, 2019	Oct. 31, 2018	Oct. 31, 2019	Oct. 31, 2018	Apr. 30, 2019
Investments Debt And Equity Securities [Abstract]
Obligations for which assets are held in trust	$ 131,500		$ 131,500		$ 122,300
Gain (Loss) on marketable securities	1,200	$ (4,800)	3,121	$ (836)
Unvested obligations under deferred compensation plans	$ 23,200		$ 23,200		$ 24,600

Amount of unrealized and realized gain (loss) on investment in marketable security, excluding other-than-temporary impairment (OTTI).

No definition available.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration