Other Balance Sheet Items - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 82,155	$ 74,430
Less: Accumulated depreciation	(21,161)	(16,288)
Total	60,994	58,142
Leasehold improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	26,658	19,074
Build-to-suit property [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	18,371	18,371
Computer equipment and software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	20,194	16,083
Furniture and fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	6,014	5,714
Construction in progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	6,446	14,440
Laboratory equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 4,472	$ 748