Income Taxes Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Apr. 30, 2017	Apr. 30, 2016	Apr. 30, 2015
Income Taxes
U.S. federal statutory income tax rate (as a percent)	34.00%	35.00%	35.00%
State and local income taxes, net of federal benefit (as a percent)	(3.00%)	(15.30%)	(84.40%)
R&D and other tax credits (as a percent)	(19.50%)	(49.90%)	(172.30%)
Valuation allowance (as a percent)	6.40%	17.10%	96.70%
Foreign rate differential	0.20%
Uncertain tax position adjustment (as a percent)			(1.90%)
Return to provision adjustments (as a percent)	(0.40%)	6.10%	78.30%
Permanent items (as a percent)	(4.00%)	(2.60%)	(5.20%)
Other (as a percent)	(1.40%)	(1.50%)	0.90%
Effective income tax rate (as a percent)	12.30%	(11.10%)	(52.90%)
Current:
Federal	$ 1,766	$ 1,804	$ 573
State	129	154	(1,292)
Current	1,895	1,958	(719)
Deferred:
Federal	(451)	(2,859)	(1,972)
State	343	3	1,689
Foreign	(35)
Deferred	(143)	(2,856)	(283)
Total income tax (benefit) expense	1,752	(898)	$ (1,002)
Deferred income tax assets:
Accrued expenses	7,074	8,238
Allowances, reserves, and other	1,801	1,330
Unrealized loss on securities	74	119
Net operating loss and credit carry-forwards	11,854	9,554
Intangibles basis	43	416
Total deferred income tax assets	20,846	19,657
Deferred income tax liabilities:
Other	29
Fixed asset basis	(428)	(395)
Total deferred income tax liabilities	(457)	(395)
Valuation allowance	(5,416)	(4,511)
Net deferred tax assets	$ 14,973	$ 14,751