Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Payroll and payroll-related expenses	$ 8,298	$ 6,611
Research and development expenses	1,772	293
Sales and marketing accruals	9,230	2,821
Accrued revenue allowances	34,445	22,195
All other	5,157	5,800
Accrued expenses and other current liabilities	$ 58,902	$ 37,720