Employee Benefit Plan - Net Periodic Pension Costs (Details) - Non-U.S. Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Plan information
Required rate of return under Belgian law, employer contributions	1.60%
Required rate of return under Belgian law, employee contributions	1.60%
Net periodic pension costs
Service cost (gross)	$ 1,397	$ 783	$ 601
Interest cost	188	243	261
Expected return on plan assets	(199)	(157)	(162)
Amortization of unrecognized actuarial gain	45
Net periodic pension cost	$ 1,431	$ 869	$ 700