SEGMENT INFORMATION (Revenues by Client Location) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 399,297	$ 377,523	$ 348,977	$ 324,651	$ 313,525	$ 298,293	$ 283,832	$ 264,482	$ 1,450,448	$ 1,160,132	$ 914,128
Reimbursable expenses and other revenues
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									17,138	12,521	9,511
United States
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									783,563	611,392	431,992
United Kingdom
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									188,995	177,194	166,418
Switzerland
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									123,281	122,919	111,686
Russia
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									61,222	40,944	36,585
Germany
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									60,158	43,621	36,403
Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									57,129	59,189	58,319
Netherlands
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									51,556	17,521	10,373
Sweden
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									32,161	23,838	10,775
Hong Kong
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									19,795	21,010	23,285
United Arab Emirates
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									12,403	3,615	0
Belgium
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									8,725	8,627	8,113
Ireland
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									8,674	5,167	5,536
China
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									7,379	4,479	817
Italy
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									6,505	4,052	2,342
Other locations
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									$ 28,902	$ 16,564	$ 11,484