SEGMENTS INFORMATION - Revenues by Client Location (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 348,977	$ 283,832	$ 673,628	$ 548,314
Reimbursable expenses and other revenues
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	4,167	3,126	8,092	5,572
United States
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	192,319	147,364	369,149	284,003
United Kingdom
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	46,112	45,652	92,706	89,212
Switzerland
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	28,119	32,596	57,215	63,448
Russia
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	13,326	10,505	26,535	19,470
Germany
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	14,144	9,736	26,238	18,475
Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	13,649	15,327	26,143	31,173
Netherlands
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	12,556	3,326	21,227	5,973
Sweden
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	8,177	6,033	15,643	10,161
Hong Kong
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	4,491	5,283	9,268	11,227
United Arab Emirates
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	3,201	0	5,249	0
China
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	1,909	413	3,493	707
Ireland
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	1,600	1,304	2,989	2,490
Italy
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	1,517	1,006	2,797	1,674
Belgium
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	1,398	2,571	2,642	4,723
Other locations
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 6,459	$ 2,716	$ 12,334	$ 5,578