Property and equipment, net - Summary of Property and Equipment Net (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Property Plant And Equipment [Line Items]
Total property and equipment, gross	$ 195,597	$ 169,399
Less accumulated depreciation and amortization	(13,239)	(12,444)
Total property and equipment, net	182,358	156,955
Building [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	161,817
Computer Equipment and Software [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	5,065	1,655
Office Equipment [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	4,196	1,427
Laboratory Equipment [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	23,191	16,305
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	116	13,697
Construction in Progress [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	$ 1,212	$ 136,315