Employee Benefit Plan - Net Periodic Pension Costs (Details) - Non-U.S. Plan $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) age	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Plan information
Required rate of return under Belgian law, employer contributions	1.60%
Required rate of return under Belgian law, employee contributions	1.60%
Benefit rights vests upon statutory retirement age | age	62
Net periodic pension costs
Service cost (gross)	$ 1,164	$ 1,281	$ 1,397
Interest cost	234	199	188
Expected return on plan assets	(242)	(327)	(199)
Amortization of unrecognized actuarial gain	(22)	18	45
Net periodic pension cost	$ 1,134	$ 1,171	$ 1,431