Financial Instruments - Financial Instruments and Balance Sheet Classification (Parenthetical) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jan. 31, 2019	Jan. 31, 2018	Jan. 31, 2019	Jan. 31, 2018	Apr. 30, 2018
Investments Debt And Equity Securities [Abstract]
Obligations for which assets are held in trust	$ 116,200		$ 116,200		$ 118,200
Unvested obligations under deferred compensation plans	23,800		23,800		$ 29,500
Gain (loss) on marketable securities	$ 2,200	$ 7,200	$ 1,330	$ 14,022