Income Taxes - Additional Information (Detail) (USD $)	12 Months Ended
	Apr. 30, 2014	Apr. 30, 2013	Apr. 30, 2012
Schedule Of Income Taxes [Line Items]
Research and development credit carry-forwards	$ 3,497,000	$ 3,169,000
Excess stock based compensation tax deductions	32,100,000
Net deferred tax assets	19,710,000	19,941,000
Income tax benefit	1,400,000	2,500,000
Deferred tax assets, valuation allowance increase	14,800,000	18,000,000
Permanently reinvested undistributed earnings	5,100,000	3,700,000	2,700,000
Permanently reinvested undistributed earnings, tax impact of distribution	1,700,000	1,300,000	900,000
Statutory income tax rate	34.00%
Unrecognized tax benefits if recognized, that would affect the effective tax rate	1,900,000
Internal Revenue Service (IRS)
Schedule Of Income Taxes [Line Items]
Net operating loss carry-forwards	190,900,000	148,500,000
Net operating loss carry-forwards, beginning expiration year	2026
Research and development credit carry-forwards	4,800,000	4,000,000
Tax credit carry-forwards, beginning expiration year	2026
State and Local Jurisdiction
Schedule Of Income Taxes [Line Items]
Net operating loss carry-forwards	83,500,000	46,200,000
Net operating loss carry-forwards, beginning expiration year	2015
Research and development credit carry-forwards	$ 1,400,000	$ 800,000
Tax credit carry-forwards, beginning expiration year	2033