Employee Benefits Plans (Details 12) - Other Postretirement Benefits - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Benefit Obligations associated with postretirement benefit plans
Benefit obligations at January 1	$ 20,965	$ 21,626
Service cost	191	215	$ 363
Interest cost	833	906	1,097
Actuarial loss (gain)	16	(338)
Benefits paid	(1,182)	(1,609)
Foreign currency exchange rate changes	418	165
Benefit obligations at December 31	$ 21,241	$ 20,965	$ 21,626