Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 45,846	$ 45,370
Less: Accumulated depreciation and amortization	(14,380)	(12,439)
Property and equipment, net	31,466	32,931
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	13,489	13,181
Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	2,852	2,832
Furniture and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,033	1,033
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	27,977	27,963
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 495	$ 361