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	$ 63,604	$ 50,856
Less: Accumulated depreciation	(14,586)	(9,695)
Total	49,018	41,161
Leasehold improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	19,151	15,051
Build-to-suit property [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	18,127	19,544
Computer equipment and software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	14,099	9,499
Construction in progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	5,778	1,360
Furniture and fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	5,714	4,667
Laboratory equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 735	$ 735