Segment Information - Property and Equipment, Net by Location (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]
Total property and equipment, net	$ 789,437	$ 698,362
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Total property and equipment, net	753,128	665,552
United Kingdom
Revenues from External Customers and Long-Lived Assets [Line Items]
Total property and equipment, net	31,279	26,921
Other
Revenues from External Customers and Long-Lived Assets [Line Items]
Total property and equipment, net	5,030	5,889
Total revenues outside of the United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Total property and equipment, net	$ 36,309	$ 32,810