Schedule of Tangible Long-Lived Assets by Geographic Area (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues from External Customers and Long-Lived Assets
Long-lived Assets	$ 913,089	$ 811,576	$ 696,567
United States
Revenues from External Customers and Long-Lived Assets
Long-lived Assets	583,236	531,425	493,300
United Kingdom
Revenues from External Customers and Long-Lived Assets
Long-lived Assets	178,021	159,689	138,546
Other countries
Revenues from External Customers and Long-Lived Assets
Long-lived Assets	$ 151,832	$ 120,462	$ 64,721