OPERATING SEGMENTS, Revenues by Client Location (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 217,781	$ 174,695	$ 417,826	$ 335,079
Reimbursable expenses and other revenues
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	2,242	1,672	4,099	3,639
United States
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	97,233	75,092	189,415	144,228
United Kingdom
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	41,423	33,176	79,988	67,420
Switzerland
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	26,933	20,833	53,284	41,365
Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	13,214	11,474	25,699	21,455
Russia
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	9,798	12,772	17,057	23,162
Germany
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	9,340	6,360	15,838	12,492
Hong Kong
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	6,062	3,210	11,272	3,210
Netherlands
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	2,308	2,344	4,354	4,462
Sweden
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	2,446	2,167	4,459	4,479
Belgium
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	1,628	899	3,153	1,540
Kazakhstan
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	1,484	1,374	2,743	2,616
Ireland
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	1,633	982	2,820	982
Other locations
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 2,037	$ 2,340	$ 3,645	$ 4,029