Property and equipment, net - Property and Equipment (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Property Plant And Equipment [Line Items]
Total property and equipment, gross	$ 104,796	$ 89,435
Less accumulated depreciation and amortization	(8,158)	(6,821)
Total Property and equipment, net	96,638	82,614
Computer Equipment and Software [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	1,292	1,259
Office Equipment [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	1,347	1,104
Laboratory Equipment [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	12,513	10,520
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	13,925	11,010
Construction in Progress [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	$ 75,719	$ 65,542