SEGMENT INFORMATION (Revenues by Client Location) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 313,525	$ 298,293	$ 283,832	$ 264,482	$ 260,253	$ 236,049	$ 217,781	$ 200,045	$ 1,160,132	$ 914,128	$ 730,027
Reimbursable expenses and other revenues
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									12,521	9,511	8,410
United States
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									605,856	427,433	318,304
United Kingdom
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									174,719	164,301	141,366
Switzerland
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									122,399	111,353	87,111
Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									58,742	57,643	49,193
Germany
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									43,216	36,089	25,740
Russia
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									40,866	36,506	48,945
Sweden
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									22,945	10,589	7,892
Hong Kong
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									20,333	23,117	13,445
Netherlands
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									16,762	9,989	8,838
Belgium
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									8,505	7,916	4,198
Ireland
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									5,152	5,437	3,667
China
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									4,445	817	0
Italy
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									3,970	2,318	1,746
Spain
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									3,406	1,028	106
Other locations
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									$ 16,295	$ 10,081	$ 11,066