OPERATING SEGMENTS, Revenues by Client Location (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 236,049	$ 192,764	$ 653,875	$ 527,843
Reimbursable expenses and other revenues
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	2,877	2,463	6,976	6,102
United States
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	110,191	84,276	299,606	228,504
United Kingdom
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	41,324	35,222	121,312	102,642
Switzerland
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	28,370	22,508	81,654	63,873
Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	14,695	13,518	40,394	34,973
Russia
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	9,469	14,351	26,526	37,513
Germany
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	10,378	6,459	26,216	18,951
Hong Kong
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	5,758	5,058	17,030	8,268
Netherlands
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	2,630	2,174	6,984	6,636
Sweden
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	3,020	1,734	7,479	6,213
Belgium
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	2,154	1,079	5,307	2,619
Kazakhstan
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	1,208	525	3,951	3,141
Ireland
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	1,554	1,448	4,374	2,430
Other locations
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 2,421	$ 1,949	$ 6,066	$ 5,978