Income Taxes - Summary of Difference Between Tax Derived by Applying Federal Statutory Income Tax Rate (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Apr. 30, 2017	Jan. 31, 2017	Oct. 31, 2016	Jul. 31, 2016	Apr. 30, 2016	Jan. 31, 2016	Oct. 31, 2015	Jul. 31, 2015	Apr. 30, 2017	Apr. 30, 2016	Apr. 30, 2015
Income Tax Disclosure [Abstract]
U.S. federal taxes at statutory rate									$ (5,232)	$ (8,603)	$ (11,257)
State tax provision									(256)	(625)	(923)
Foreign tax rate differentials									(95)	(108)	(206)
Research and development credit									(912)	(1,473)	(1,972)
Stock options									2,348	3,480	506
Nondeductible legal expenses									0	0	200
Permanent differences and other									167	0	(8)
Return to provision adjustments									(219)	9	0
Change in valuation allowance									4,752	7,358	13,714
Provision for (benefit from) income taxes	$ 203	$ 123	$ 92	$ 135	$ 165	$ (163)	$ 124	$ (88)	$ 553	$ 38	$ 54