SEGMENTS INFORMATION - Revenues by Client Location (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 283,832	$ 217,781	$ 548,314	$ 417,826
Reimbursable expenses and other revenues
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	3,126	2,242	5,572	4,099
United States
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	145,882	97,233	281,440	189,415
United Kingdom
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	45,016	41,423	88,006	79,988
Switzerland
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	32,456	26,933	63,221	53,284
Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	15,173	13,214	30,874	25,699
Russia
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	10,484	9,798	19,436	17,057
Germany
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	9,663	9,340	18,369	15,838
Hong Kong
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	5,091	6,062	10,845	11,272
Sweden
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	5,888	2,446	9,948	4,459
Netherlands
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	3,165	2,308	5,712	4,354
Belgium
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	2,529	1,628	4,653	3,153
Ireland
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	1,296	1,633	2,480	2,820
Italy
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	993	680	1,629	1,093
Kazakhstan
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	650	1,484	1,392	2,743
Other locations
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 2,420	$ 1,357	$ 4,737	$ 2,552