Income Taxes - Additional Information (Detail) - USD ($)	9 Months Ended	12 Months Ended
	Dec. 31, 2017	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018	Apr. 01, 2017
Income Tax Contingency [Line Items]
U.S. statutory federal income tax rate, percent	35.00%		21.00%	31.60%
Cumulative effect adjustment related to unrecognized excess tax benefits on share based compensation					$ 58,700,000
Valuation allowance		$ 42,621,000	$ 33,499,000
Impact on effective tax rate		72,600,000
Accrued interest or penalties associated with uncertain tax positions		0	$ 0
Income tax expense				$ 5,300,000
United States [Member]
Income Tax Contingency [Line Items]
Operating loss carryforwards		$ 831,900,000
Operating loss carryforwards expiration beginning year		2021
Tax years subject to examination		2017 and thereafter
United States [Member] | Research & Development [Member]
Income Tax Contingency [Line Items]
Tax credit carryforward amount		$ 189,900,000
United States [Member] | Research & Development [Member] | Earliest Tax Year [Member]
Income Tax Contingency [Line Items]
Tax credit carryforwards expiration beginning year		2026
State [Member]
Income Tax Contingency [Line Items]
Operating loss carryforwards		$ 603,900,000
Operating loss carryforwards expiration beginning year		2021
Tax years subject to examination		2017 and thereafter
State [Member] | Research & Development [Member]
Income Tax Contingency [Line Items]
Tax credit carryforward amount		$ 162,600,000
State [Member] | Research & Development [Member] | Earliest Tax Year [Member]
Income Tax Contingency [Line Items]
Tax credit carryforwards expiration beginning year		2021
Foreign [Member]
Income Tax Contingency [Line Items]
Tax credit carryforward amount		$ 1,900,000
Tax years subject to examination		2016 and thereafter
Foreign [Member] | Earliest Tax Year [Member]
Income Tax Contingency [Line Items]
Tax credit carryforwards expiration beginning year		2021