Unaudited Quarterly Financial Information - Summary of Unaudited Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Apr. 30, 2020	Jan. 31, 2020	Oct. 31, 2019	Jul. 31, 2019	Apr. 30, 2019	Jan. 31, 2019	Oct. 31, 2018	Jul. 31, 2018	Apr. 30, 2020	Apr. 30, 2019	Apr. 30, 2018
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 3,824	$ 1,577	$ 3,920	$ 3,369	$ 2,926	$ 3,890	$ 5,592	$ 3,718	$ 12,690	$ 16,127	$ 8,394
Operating expenses	12,781	14,301	13,209	12,933	14,186	10,550	10,485	10,727	53,223	45,947	27,099
Net loss	$ (6,585)	$ (9,291)	$ (5,903)	$ (7,338)	$ (8,526)	$ (3,956)	$ (3,304)	$ (5,030)	$ (29,116)	$ (20,816)	$ (15,805)
Net loss per share	$ (0.37)	$ (0.52)	$ (0.33)	$ (0.42)	$ (0.49)	$ (0.23)	$ (0.22)	$ (0.47)	$ (1.64)	$ (1.38)	$ (1.53)