SEGMENTS INFORMATION - Revenues by Client Location (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 324,651	$ 264,482
Reimbursable expenses and other revenues
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	3,925	2,446
United States
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	176,830	136,639
United Kingdom
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	46,594	43,560
Switzerland
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	29,096	30,852
Russia
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	13,209	8,965
Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	12,494	15,846
Germany
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	12,094	8,739
Netherlands
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	8,671	2,647
Sweden
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	7,466	4,128
Hong Kong
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	4,777	5,944
United Arab Emirates
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	2,048	0
China
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	1,584	294
Ireland
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	1,389	1,186
Italy
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	1,280	668
Denmark
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	1,253	410
Other locations
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 5,866	$ 4,604