Pensions - Components of Net Periodic Pension Costs (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
U.S. Plans
Components of net periodic pension costs of the plans
Service cost	$ 218	$ 220	$ 480	$ 478
Interest cost	1,096	1,127	2,206	2,297
Expected return on plan assets	(1,474)	(1,507)	(2,947)	(3,017)
Net actuarial loss amortization	748	720	1,494	1,504
Settlement		680		680
Net periodic pension cost (benefit)	588	1,240	1,233	1,942
European Plans
Components of net periodic pension costs of the plans
Service cost	126	94	248	186
Interest cost	238	300	468	597
Expected return on plan assets	(325)	(377)	(639)	(752)
Amortization of prior service credit	(2)		(4)
Net actuarial loss amortization	110	59	217	117
Net periodic pension cost (benefit)	$ 147	$ 76	$ 290	$ 148