Segment Information - Property and Equipment, Net by Location (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Revenues from External Customers and Long-Lived Assets [Line Items]
Property and equipment, net	$ 812,005	$ 789,437
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Property and equipment, net	778,157	753,128
United Kingdom
Revenues from External Customers and Long-Lived Assets [Line Items]
Property and equipment, net	30,496	31,279
Other
Revenues from External Customers and Long-Lived Assets [Line Items]
Property and equipment, net	3,352	5,030
Total revenues outside of the United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Property and equipment, net	$ 33,848	$ 36,309