Income Taxes - Effective Income Tax Rate (Detail) - USD ($) $ in Thousands	12 Months Ended
	Apr. 30, 2018	Apr. 30, 2017	Apr. 30, 2016
Income Tax Disclosure [Abstract]
Computed "expected" income tax expense	$ 4,763	$ 7,565	$ 5,145
Increase (decrease) in income taxes resulting from:
State income taxes, net of federal income tax effect	891	592	435
Research and development credits	(493)	(520)	(694)
Change in valuation allowance for deferred tax assets	4	(293)	(2)
Tax contingencies	(13)	(42)	(13)
NQ stock options granted	102	106	26
Rate change - tax reform	(1,206)
Other, net, including permanent items	(386)	(414)	(439)
Income tax expense	$ 3,662	$ 6,994	$ 4,458