Revenues - Disaggregation of Revenues by Geography (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation Of Revenue [Line Items]
Disaggregation of revenue	$ 421,929	$ 407,699
United States [Member]
Disaggregation Of Revenue [Line Items]
Disaggregation of revenue	302,703	296,836
United Kingdom [Member]
Disaggregation Of Revenue [Line Items]
Disaggregation of revenue	32,011	27,543
Asia-Pacific and Japan [Member]
Disaggregation Of Revenue [Line Items]
Disaggregation of revenue	28,988	26,449
Europe, excluding United Kingdom [Member]
Disaggregation Of Revenue [Line Items]
Disaggregation of revenue	27,808	24,876
Canada [Member]
Disaggregation Of Revenue [Line Items]
Disaggregation of revenue	18,318	20,628
Americas, excluding United States and Canada [Member]
Disaggregation Of Revenue [Line Items]
Disaggregation of revenue	$ 12,101	$ 11,367