Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 49,508	$ 22,398
Less: Accumulated depreciation	(8,433)	(5,363)
Total	41,075	17,035
Build-to-suit property [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	19,030
Leasehold improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	14,849	12,034
Computer equipment and software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	7,624	4,503
Furniture and fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	4,660	3,981
Construction in progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	2,631	1,177
Laboratory equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 714	$ 703