Net Periodic Pension Cost (Details) - UNITED KINGDOM - Foreign Pension Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Components of the net periodic pension cost:
Interest cost	$ 1,232	$ 1,340	$ 1,589
Expected return on plan assets	(1,566)	(1,844)	(1,958)
Amortization of actuarial losses	726	875	374
Net periodic pension cost	$ 392	$ 371	$ 5