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Employee Benefit Plan - Net Periodic Pension Costs (Details) - Non-U.S. Plan
$ in Thousands
 12 Months Ended
Dec. 31, 2018
USD ($)
age
Dec. 31, 2017
USD ($)
Dec. 31, 2016
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) $ 1,281 $ 1,397 $ 783
Interest cost 199 188 243
Expected return on plan assets (327) (199) (157)
Amortization of unrecognized actuarial gain 18 45  
Net periodic pension cost $ 1,171 $ 1,431 $ 869