Income Taxes (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Apr. 30, 2013	Apr. 30, 2012	Apr. 30, 2011
Reconciliation of income tax expense computed using the U.S. federal statutory rates to actual income tax expense
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)	(9.60%)	(0.30%)	1.10%
R&D and other tax credits (as a percent)	(29.60%)	(3.40%)	(11.30%)
Other (as a percent)	7.40%	(1.20%)	(0.50%)
Effective income tax rate (as a percent)	3.20%	30.10%	24.30%
Current:
Federal	$ (3,818)	$ 12,814	$ 8,660
State	334	1,651	641
Current	(3,484)	14,465	9,301
Deferred:
Federal	5,178	(187)	859
State	(1,347)	(1,134)	(1,836)
Deferred	3,831	(1,321)	(977)
Change in valuation allowance		(57)	(5)
Total income tax expense	347	13,087	8,319
Deferred income tax assets:
Accrued expenses	6,266	9,697
Allowances, reserves, and other	3,050	763
Capital loss and credit carry-forwards	4,908	4,508
Unrealized loss on securities		454
Total deferred income tax assets	14,224	15,422
Deferred income tax liabilities:
Unrealized gain on securities	(1,811)
Tax over book depreciation	(1,641)	(836)
Total deferred income tax liabilities	(3,452)	(836)
Net deferred tax assets	$ 10,772	$ 14,586