Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 53,257	$ 50,856
Less: Accumulated depreciation	(11,226)	(9,695)
Total	42,031	41,161
Build-to-suit property [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	20,071	19,544
Leasehold improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	15,002	15,051
Computer equipment and software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	9,976	9,499
Furniture and fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	4,667	4,667
Construction in progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	2,806	1,360
Laboratory equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 735	$ 735