Accrued Expenses - Summary of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Payroll and employee-related costs	$ 15,271	$ 13,044
Professional services	3,208	2,221
Property and equipment purchases	2,378	115
Other	7,389	5,198
Accrued expenses	$ 28,246	$ 20,578