Segment Information - Revenue Classified by Major Geographic Areas (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenue	$ 264,973	$ 226,220	$ 215,884	$ 213,261	$ 218,945	$ 211,804	$ 212,074	$ 214,956	$ 920,338	$ 857,779	$ 897,801
Americas
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenue									880,602	815,231	853,221
Europe/Middle East
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenue									36,364	39,048	40,603
Asia
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenue									$ 3,372	$ 3,500	$ 3,977