Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 44,260	$ 41,305
Less: Accumulated depreciation and amortization	(10,536)	(5,180)
Property and equipment, net	33,724	36,125
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	12,544	10,711
Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	2,555	1,335
Furniture and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,033	1,010
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	27,896	27,807
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 232	$ 442