Income Taxes - Schedule of Effective Federal Statutory Rate and Provision for Income Taxes (Detail)	12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Income Tax Disclosure [Abstract]
Federal statutory rate	21.00%	30.80%	34.00%
Effect of:
State taxes, net of federal benefits	5.00%	3.20%	2.40%
Stock-based compensation	35.20%	82.40%	(1.80%)
Research and development credits, net of ASC 740-10	8.70%	6.50%	3.50%
Tax Cuts and Jobs Act	0.00%	(71.00%)	0.00%
Permanent items	(3.90%)	(2.10%)	0.00%
Foreign taxes	(0.30%)	(0.60%)	0.00%
Business combination	1.40%	0.00%	0.00%
Other	0.70%	0.60%	0.40%
Valuation allowance	(69.50%)	(52.00%)	(38.90%)
Effective tax rate	(1.70%)	(2.20%)	(0.40%)