Segment Operating Results - Revenues Derived from Clients by Geographical Areas (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Net Revenues	$ 2,063,425	$ 1,635,517	$ 1,427,410
United States [Member]
Segment Reporting Information [Line Items]
Net Revenues	1,591,883	1,199,231	1,057,633
Europe and Other [Member]
Segment Reporting Information [Line Items]
Net Revenues	438,602	422,271	337,957
Latin America [Member]
Segment Reporting Information [Line Items]
Net Revenues	$ 32,940	$ 14,015	$ 31,820