OPERATING SEGMENTS, Revenues by Client Location (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 200,045	$ 160,384
Reimbursable expenses and other revenues
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	1,857	1,967
United States
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	92,182	69,136
United Kingdom
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	38,565	34,244
Switzerland
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	26,351	20,532
Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	12,485	9,981
Russia
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	7,259	10,390
Germany
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	6,498	6,132
Hong Kong
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	5,210	0
Netherlands
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	2,046	2,118
Sweden
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	2,013	2,312
Belgium
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	1,525	641
Kazakhstan
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	1,259	1,242
Ireland
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	1,187	0
Other locations
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 1,608	$ 1,689