Property and Equipment, net - Summary of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	$ 31,702	$ 25,875
Accumulated depreciation	(10,628)	(7,375)
Total property and equipment, net	21,074	18,500
Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	585	443
Furniture, Fixtures and Other [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	2,640	2,453
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	12,212	8,964
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	16,210	13,776
Construction Work in Process [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	$ 55	$ 239