Effective Income Tax Rate (Detail) - USD ($) $ in Thousands	12 Months Ended
	Apr. 30, 2017	Apr. 30, 2016	Apr. 30, 2015
Income Tax Disclosure [Abstract]
Computed "expected" income tax expense	$ 7,565	$ 5,145	$ 3,641
Increase (decrease) in income taxes resulting from:
State income taxes, net of federal income tax effect	592	435	269
Research and development credits	(520)	(694)	(516)
Change in valuation allowance for deferred tax assets	(293)	(2)	(5)
Tax contingencies	(42)	(13)	(955)
Other, net, including permanent items	(308)	(413)	(160)
Income tax expense	$ 6,994	$ 4,458	$ 2,274