Segment Information - Revenue Classified by Major Geographic Areas (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenue	$ 243,254	$ 237,814	$ 242,556	$ 234,530	$ 264,973	$ 226,220	$ 215,884	$ 213,261	$ 958,154	$ 920,338	$ 857,779
Americas
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenue									925,127	880,602	815,231
Europe/Middle East
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenue									29,292	36,364	39,048
Asia
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenue									$ 3,735	$ 3,372	$ 3,500