Segment and Geographical Information - Revenues based on Geographical Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues, net	$ 192,345	$ 171,200	$ 564,435	$ 508,714
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues, net	158,501	137,047	462,676	407,272
United Kingdom
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues, net	26,824	27,993	81,857	84,284
Rest of World
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues, net	7,020	6,160	19,902	17,158
Total Non-United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues, net	$ 33,844	$ 34,153	$ 101,759	$ 101,442