Accrued Expenses and Other Current Liabilities - Summary of Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Payroll and employee-related costs	$ 19,630	$ 13,044
Professional services	7,935	2,221
Property and equipment purchases	688	115
Other	8,492	5,198
Total accrued expenses and other current liabilities	$ 36,745	$ 20,578