Income Taxes Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Apr. 30, 2016	Apr. 30, 2015	Apr. 30, 2014
Income Taxes
U.S. federal statutory income tax rate (as a percent)	35.00%	35.00%	35.00%
State and local income taxes, net of federal benefit (as a percent)	(15.30%)	(84.40%)	(17.00%)
R&D and other tax credits (as a percent)	(49.90%)	(172.30%)	(21.50%)
Valuation allowance (as a percent)	17.10%	96.70%	8.70%
Uncertain tax position adjustment (as a percent)		(1.90%)	4.40%
Return to provision adjustments (as a percent)	6.10%	78.30%	(0.10%)
Permanent items (as a percent)	(2.60%)	(5.20%)	(1.30%)
Other (as a percent)	(1.50%)	0.90%	(0.30%)
Effective income tax rate (as a percent)	(11.10%)	(52.90%)	7.90%
Current:
Federal	$ 1,804	$ 573	$ 4,307
State	154	(1,292)	(1,879)
Current	1,958	(719)	2,428
Deferred:
Federal	(2,859)	(1,972)	(1,694)
State	3	1,689	444
Deferred	(2,856)	(283)	(1,250)
Total income tax (benefit) expense	(898)	(1,002)	$ 1,178
Deferred income tax assets:
Accrued expenses	8,238	8,442
Allowances, reserves, and other	1,330	1,543
Unrealized loss on securities	119
Net operating loss and credit carry-forwards	9,554	5,692
Intangibles basis	416	464
Total deferred income tax assets	19,657	16,141
Deferred income tax liabilities:
Unrealized gain on securities		(237)
Fixed asset basis	(395)	(86)
Total deferred income tax liabilities	(395)	(323)
Valuation allowance	(4,511)	(3,127)
Net deferred tax assets	$ 14,751	$ 12,691