Net Periodic Pension Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Components of the net periodic pension cost:
Interest cost	$ 991	$ 1,232	$ 1,340
Expected return on plan assets	(1,665)	(1,566)	(1,844)
Amortization of actuarial losses	1,045	726	875
Net periodic pension cost	$ 371	$ 392	$ 371
Defined Benefit Plan, Sponsor Location [Extensible List]	us-gaap:ForeignPlanMember	us-gaap:ForeignPlanMember	us-gaap:ForeignPlanMember