Balance Sheets Components - Property and Equipment (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	$ 73,714	$ 61,921
Less: accumulated depreciation	(7,365)	(5,472)
Total property and equipment, net	66,349	56,449	[1]
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	31,516	29,924
Laboratory equipment
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	13,967	13,117
Computers equipment and purchased software
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	3,774	3,726
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	2,804	2,764
Construction in progress
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	$ 21,653	$ 12,390

[1] The balance sheet as of December 31, 2019 is derived from the audited financial statements as of that date.