SEGMENTS INFORMATION - Revenues by Client Location (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 377,523	$ 298,293	$ 1,051,151	$ 846,607
Reimbursable expenses and other revenues
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	4,134	3,159	12,226	8,731
United States
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	202,364	155,241	571,513	439,244
United Kingdom
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	48,471	44,179	141,177	133,391
Switzerland
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	33,185	31,638	90,400	95,086
Germany
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	16,053	12,451	42,291	30,926
Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	15,711	14,589	41,854	45,762
Russia
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	14,079	9,861	40,614	29,331
Netherlands
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	14,118	4,539	35,345	10,512
Sweden
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	8,491	6,506	24,134	16,667
Hong Kong
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	5,185	4,991	14,453	16,218
United Arab Emirates
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	3,282	1,788	8,531	1,788
Ireland
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	2,620	1,318	5,609	3,808
China
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	1,939	2,071	5,432	2,778
Belgium
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	2,785	2,699	5,427	7,422
Italy
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	1,654	1,125	4,451	2,799
Other locations
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 7,586	$ 5,297	$ 19,920	$ 10,875