U.K. Pension Plan Amount Recognized in Balance Sheet (Details) - Pension Plan [Member] - Foreign Plan [Member] - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Accrued expenses	$ 13,879	$ 9,047
Other long-term liabilities	86,404	84,986
Defined benefit plan, liabilities recognized in balance sheet	$ (100,283)	$ (94,033)