Summary of Significant Accounting Policies - Schedule of Basic and Diluted Net Loss Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Jan. 31, 2018	Jan. 31, 2017	Jan. 31, 2018	Jan. 31, 2017
Earnings Per Share Basic [Line Items]
Net loss	$ (5,234)	$ (7,644)	$ (15,148)	$ (14,401)
Loss available to common shareholders for the purpose of calculating basic and diluted net loss per share	$ (5,234)	$ (7,855)	$ (15,148)	$ (16,573)
Weighted average common shares, basic and diluted	10,788,556	7,657,874	10,168,520	3,013,073
Net loss per share, basic and diluted	$ (0.49)	$ (1.03)	$ (1.49)	$ (5.50)
Series A Preferred Shares [Member]
Earnings Per Share Basic [Line Items]
Less: dividend		$ 91		$ 935
Series B Preferred Shares [Member]
Earnings Per Share Basic [Line Items]
Less: dividend		$ 120		$ 1,237