Financial Instruments - Financial Instruments and Balance Sheet Classification (Parenthetical) (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Jan. 31, 2020	Jan. 31, 2019	Jan. 31, 2020	Jan. 31, 2019	Apr. 30, 2019
Investments Debt And Equity Securities [Abstract]
Obligations for which assets are held in trust	$ 135,700		$ 135,700		$ 122,300
Gain on marketable securities	5,200	$ 2,200	8,341	$ 1,330
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.

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