Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Payables and Accruals [Abstract]
Payroll and other employee related costs	$ 11,106	$ 19,014
Professional services and consulting and other vendor fees	16,176	17,461
Unrecognized tax benefits	1,930	1,913
Sales commissions	3,689	6,239
Contingent earn-out	1,885	2,372
Restructuring	72	977	$ 2,338
Other	1,039	2,686
Total	$ 35,897	$ 50,662