OPERATING SEGMENTS (Revenues by Client Location) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 202,184	$ 192,764	$ 174,695	$ 160,384	$ 157,585	$ 140,150	$ 133,184	$ 124,198	$ 730,027	$ 555,117	$ 433,799
Reimbursable expenses and other revenues
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									8,410	7,626	6,417
United States
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									318,304	247,979	197,593
United Kingdom
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									141,366	108,892	98,346
Switzerland
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									87,111	51,941	30,120
Russia
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									48,945	53,328	47,507
Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									49,193	33,759	9,256
Germany
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									25,740	20,261	16,391
China
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									13,445	0	0
Netherlands
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									8,838	7,719	3,127
Sweden
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									7,892	5,742	4,913
Kazakhstan
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									5,238	9,886	11,352
Other locations
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									$ 15,545	$ 7,984	$ 8,777