Merck Arrangement - Additional Information (Detail) - Merck [Member] - USD ($) $ in Millions	1 Months Ended	3 Months Ended	9 Months Ended
	Nov. 30, 2017	Jan. 31, 2020	Jan. 31, 2020	Apr. 30, 2020
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized		$ 1.6	$ 8.9
Remaining Unsatisfied Performance Obligations [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue		$ 3.8	$ 3.8
Remaining Unsatisfied Performance Obligations [Member] | Forecast [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue				$ 3.8
KalVista Pharmaceuticals Limited [Member] | Merck Option Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Agreement commencement date			Oct. 06, 2017
Non-refundable upfront fee	$ 37.0