Segment and Geographical Information - Revenues based on Geographical Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues, net	$ 189,057	$ 170,478	$ 372,090	$ 337,514
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues, net	153,894	136,151	304,175	270,225
United Kingdom
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues, net	28,951	28,863	55,033	56,291
Rest of World
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues, net	6,212	5,464	12,882	10,998
Non-US
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues, net	$ 35,163	$ 34,327	$ 67,915	$ 67,289