Property and equipment, net - Property and Equipment (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Property Plant And Equipment [Line Items]
Total property and equipment	$ 89,435	$ 19,141
Less accumulated depreciation and amortization	(6,821)	(3,401)
Property and equipment, net	82,614	15,740
Computer Equipment and Software [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	1,259	814
Office Equipment [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	1,104	786
Laboratory Equipment [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	10,520	7,223
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	11,010	$ 10,318
Construction in Progress [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	$ 65,542