Investments - Additional Information (Detail) - USD ($)	12 Months Ended
	Apr. 30, 2017	Apr. 30, 2016	Apr. 30, 2015
Investments, Debt and Equity Securities [Abstract]
Carrying value of investment	$ 23,787,000	$ 28,881,000
Trading investments	4,455,000	7,924,000
Net unrealized holding gain (losses)	1,300,000	(110,000)	$ (405,000)
Net realized holding losses	$ (937,000)	$ (878,000)	$ (228,000)