Property and equipment, net - Property and Equipment (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Property Plant And Equipment [Line Items]
Total property and equipment	$ 169,399	$ 89,435
Less accumulated depreciation and amortization	(12,444)	(6,821)
Property and equipment, net	156,955	82,614
Computer Equipment and Software [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	1,655	1,259
Office Equipment [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	1,427	1,104
Laboratory Equipment [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	16,305	10,520
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	13,697	11,010
Construction in Progress [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	$ 136,315	$ 65,542