OPERATING SEGMENTS - Revenues by Client Location (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 264,482	$ 200,045
Reimbursable expenses and other revenues
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	2,446	1,857
Russia
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	135,558	92,182
United Kingdom
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	42,990	38,565
Switzerland
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	30,765	26,351
Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	15,701	12,485
Russia
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	8,952	7,259
Germany
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	8,706	6,498
Hong Kong
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	5,754	5,210
Netherlands
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	2,547	2,046
Sweden
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	4,060	2,013
Belgium
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	2,124	1,525
Kazakhstan
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	742	1,259
Ireland
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	1,184	1,187
Other locations
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 2,953	$ 1,608