Employee Benefit Plan - Net Unfunded Status (Details) - Non-U.S. Plan - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net unfunded status of pension plan
Fair value of plan assets	$ 12,390	$ 11,024	$ 9,264
Projected benefit obligation	(18,308)	(16,690)	$ (12,674)
Net unfunded benefit obligation	$ (5,918)	$ (5,666)