Segment Information - Revenue Classified by Major Geographic Areas (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenue	$ 237,814	$ 226,220	$ 714,900	$ 655,365
Americas [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenue	230,486	217,900	690,569	625,615
Europe/Middle East
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenue	6,500	7,655	21,245	27,019
Asia
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenue	$ 828	$ 665	$ 3,086	$ 2,731