Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 5,220	$ 1,547
Less: Accumulated depreciation and amortization	(677)	(283)
Property and equipment, net	4,543	1,264
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	2,700	1,260
Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	292	115
Furniture and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	236	58
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,856	$ 114
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 136