Net Loss Per Share Applicable to Common Stockholders - Computations of Loss per Share Applicable to Common Stockholders (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		3 Months Ended		9 Months Ended
		Jan. 31, 2017	Jan. 31, 2016	Jan. 31, 2017	Jan. 31, 2016
Schedule Of Computation Of Basic And Diluted Earnings Per Common Share [Line Items]
Net loss		$ (2,728)	$ (3,114)	$ (11,967)	$ (18,210)
Basic and diluted loss per share		$ (0.03)	$ (0.04)	$ (0.14)	$ (0.23)
Basic and diluted weighted average number of shares outstanding		83,348	81,096	82,830	80,649
Outstanding stock options
Potentially dilutive securities (1):
Outstanding stock options	[1]	360	168	279	224
Restricted shares
Potentially dilutive securities (1):
Outstanding stock options	[1]	963	17	677	50

[1] The impact of potentially dilutive securities on earnings per share is anti-dilutive in a period of net loss.