Property and equipment, net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Property Plant And Equipment [Line Items]
Depreciation and amortization expense	$ 5,900	$ 3,700	$ 2,300
Property and equipment, gross	169,399	89,435
Construction cost incurred by landlord	120,140	$ 61,901
Construction In Progress 60 Binney Street [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	126,900
Construction cost incurred by landlord	$ 120,100