Loss Per Share Applicable to Common Stockholders (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Oct. 31, 2012	Oct. 31, 2011	Oct. 31, 2012	Oct. 31, 2011
Earnings Per Share, Basic and Diluted
Net loss	$ (11,242)	$ (5,851)	$ (29,781)	$ (11,387)
Accretion of redeemable convertible preferred stock		(12)		(25)
Loss applicable to common stockholders	$ (11,242)	$ (5,863)	$ (29,781)	$ (11,412)
Loss per share, basic and diluted	$ (0.16)	$ (0.30)	$ (0.45)	$ (0.60)
Weighted average number of shares	69,846	19,421	66,203	19,119
Potentially dilutive securities (1):
Outstanding stock options	7,299	5,583	7,828	5,057
Redeemable convertible preferred shares				27,897