Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Payables and Accruals [Abstract]
Payroll and other employee related costs	$ 16,942	$ 16,431
Professional services and consulting and other vendor fees	12,688	15,674
Unrecognized tax benefits	2,332	4,924
Sales commissions	5,719	5,259
Contingent earn-out	1,800	0
Restructuring	1,249	2,338	$ 2,551
Other	394	3,385
Total	$ 41,124	$ 48,011