Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 50,856	$ 22,398
Less: Accumulated depreciation	(9,695)	(5,363)
Total	41,161	17,035
Build-to-suit property [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	19,544
Leasehold improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	15,051	12,034
Computer equipment and software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	9,499	4,503
Furniture and fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	4,667	3,981
Construction in progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,360	1,177
Laboratory equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 735	$ 703