Financial Instruments - Financial Instruments and Balance Sheet Classification (Parenthetical) (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Oct. 31, 2018	Oct. 31, 2017	Oct. 31, 2018	Oct. 31, 2017	Apr. 30, 2018
Investments Debt And Equity Securities [Abstract]
Obligations for which assets are held in trust	$ 115,900		$ 115,900		$ 118,200
Unvested obligations under deferred compensation plans	24,200		24,200		$ 29,500
Gain (loss) on marketable securities	$ (4,800)	$ 3,400	$ (836)	$ 6,797

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