Segment and Geographical Information - Revenues based on Geographical Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues, net	$ 210,112	$ 189,057	$ 417,085	$ 372,090
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues, net	174,087	153,894	345,285	304,175
United Kingdom
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues, net	27,480	28,951	55,496	55,033
Rest of World
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues, net	8,545	6,212	16,304	12,882
Total Non-United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues, net	$ 36,025	$ 35,163	$ 71,800	$ 67,915