Merck Arrangement - Additional Information (Detail) $ in Millions		1 Months Ended	12 Months Ended
	Oct. 06, 2017 PerformanceObligation	Nov. 30, 2017 USD ($)	Apr. 30, 2020 USD ($)	Apr. 30, 2019 USD ($)	Apr. 30, 2018 USD ($)
Research and Development Services for KVD001 and Oral DME [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of performance obligations | PerformanceObligation	2
Merck [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized			$ 12.7	$ 16.1	$ 8.0
Merck [Member] | Remaining Unsatisfied Performance Obligations [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue			$ 3.8
KalVista Pharmaceuticals Limited [Member] | Merck [Member] | Option Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Agreement commencement date			Oct. 06, 2017
Non-refundable upfront fee		$ 37.0