OPERATING SEGMENTS (Revenues by Client Location) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 260,253	$ 236,049	$ 217,781	$ 200,045	$ 202,184	$ 192,764	$ 174,695	$ 160,384	$ 914,128	$ 730,027	$ 555,117
Reimbursable expenses and other revenues
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									9,511	8,410	7,626
United States
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									427,433	318,304	247,979
United Kingdom
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									164,301	141,366	108,892
Switzerland
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									111,353	87,111	51,941
Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									57,643	49,193	33,759
Russia
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									36,506	48,945	53,328
Germany
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									36,089	25,740	20,261
Hong Kong
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									23,117	13,445	0
Sweden
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									10,589	7,892	5,742
Netherlands
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									9,989	8,838	7,719
Belgium
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									7,916	4,198	0
Ireland
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									5,437	3,667	291
Kazakhstan
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									4,910	5,238	9,886
Other locations
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									$ 9,334	$ 7,680	$ 7,693