SEGMENTS INFORMATION - Revenues by Client Location (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 445,647	$ 348,977	$ 869,795	$ 673,628
United States
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	246,831	192,319	470,514	369,149
United Kingdom
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	50,103	46,112	101,833	92,706
Switzerland
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	34,268	28,119	69,872	57,215
Germany
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	20,111	14,144	39,599	26,238
Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	17,620	13,649	33,383	26,143
Netherlands
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	17,254	12,556	34,903	21,227
Russia
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	16,909	13,326	36,321	26,535
Other locations
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 42,551	$ 28,752	$ 83,370	$ 54,415