NATURE OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Change in Presentation of Certain Financial Information) (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
Effect of Reclassification Adjustments [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
Effect of Reclassification Adjustments [Line Items]
Revenues									17,138	12,521	9,511
United States
Effect of Reclassification Adjustments [Line Items]
Revenues									783,563	611,392	431,992
United Kingdom
Effect of Reclassification Adjustments [Line Items]
Revenues									188,995	177,194	166,418
Switzerland
Effect of Reclassification Adjustments [Line Items]
Revenues									123,281	122,919	111,686
Canada
Effect of Reclassification Adjustments [Line Items]
Revenues									57,129	59,189	58,319
Germany
Effect of Reclassification Adjustments [Line Items]
Revenues									60,158	43,621	36,403
Russia
Effect of Reclassification Adjustments [Line Items]
Revenues									61,222	40,944	36,585
Sweden
Effect of Reclassification Adjustments [Line Items]
Revenues									32,161	23,838	10,775
Hong Kong
Effect of Reclassification Adjustments [Line Items]
Revenues									19,795	21,010	23,285
Netherlands
Effect of Reclassification Adjustments [Line Items]
Revenues									51,556	17,521	10,373
Belgium
Effect of Reclassification Adjustments [Line Items]
Revenues									8,725	8,627	8,113
Ireland
Effect of Reclassification Adjustments [Line Items]
Revenues									8,674	5,167	5,536
China
Effect of Reclassification Adjustments [Line Items]
Revenues									7,379	4,479	817
Italy
Effect of Reclassification Adjustments [Line Items]
Revenues									6,505	4,052	2,342
United Arab Emirates
Effect of Reclassification Adjustments [Line Items]
Revenues									12,403	3,615	0
Other locations
Effect of Reclassification Adjustments [Line Items]
Revenues									$ 28,902	16,564	11,484
As Reported
Effect of Reclassification Adjustments [Line Items]
Revenues										$ 1,160,132	$ 914,128
Percent of Revenue										100.00%	100.00%
As Reported | Reimbursable expenses and other revenues
Effect of Reclassification Adjustments [Line Items]
Percent of Revenue										1.10%	1.00%
As Reported | United States
Effect of Reclassification Adjustments [Line Items]
Revenues										$ 605,856	$ 427,433
Percent of Revenue										52.20%	46.80%
As Reported | United Kingdom
Effect of Reclassification Adjustments [Line Items]
Revenues										$ 174,719	$ 164,301
Percent of Revenue										15.10%	18.00%
As Reported | Switzerland
Effect of Reclassification Adjustments [Line Items]
Revenues										$ 122,399	$ 111,353
Percent of Revenue										10.60%	12.20%
As Reported | Canada
Effect of Reclassification Adjustments [Line Items]
Revenues										$ 58,742	$ 57,643
Percent of Revenue										5.10%	6.30%
As Reported | Germany
Effect of Reclassification Adjustments [Line Items]
Revenues										$ 43,216	$ 36,089
Percent of Revenue										3.70%	3.90%
As Reported | Russia
Effect of Reclassification Adjustments [Line Items]
Revenues										$ 40,866	$ 36,506
Percent of Revenue										3.50%	4.00%
As Reported | Sweden
Effect of Reclassification Adjustments [Line Items]
Revenues										$ 22,945	$ 10,589
Percent of Revenue										2.00%	1.20%
As Reported | Hong Kong
Effect of Reclassification Adjustments [Line Items]
Revenues										$ 20,333	$ 23,117
Percent of Revenue										1.80%	2.50%
As Reported | Netherlands
Effect of Reclassification Adjustments [Line Items]
Revenues										$ 16,762	$ 9,989
Percent of Revenue										1.40%	1.10%
As Reported | Belgium
Effect of Reclassification Adjustments [Line Items]
Revenues										$ 8,505	$ 7,916
Percent of Revenue										0.70%	0.90%
As Reported | Ireland
Effect of Reclassification Adjustments [Line Items]
Revenues										$ 5,152	$ 5,437
Percent of Revenue										0.40%	0.60%
As Reported | China
Effect of Reclassification Adjustments [Line Items]
Revenues										$ 4,445	$ 817
Percent of Revenue										0.40%	0.10%
As Reported | Italy
Effect of Reclassification Adjustments [Line Items]
Revenues										$ 3,970	$ 2,318
Percent of Revenue										0.30%	0.30%
As Reported | United Arab Emirates
Effect of Reclassification Adjustments [Line Items]
Revenues										$ 3,486
Percent of Revenue										0.30%
As Reported | Other locations
Effect of Reclassification Adjustments [Line Items]
Revenues										$ 16,215	$ 11,109
Percent of Revenue										1.40%	1.10%
After Reclassification
Effect of Reclassification Adjustments [Line Items]
Revenues										$ 1,160,132	$ 914,128
Percent of Revenue										100.00%	100.00%
After Reclassification | Reimbursable expenses and other revenues
Effect of Reclassification Adjustments [Line Items]
Revenues										$ 0	$ 0
Percent of Revenue										0.00%	0.00%
After Reclassification | United States
Effect of Reclassification Adjustments [Line Items]
Revenues										$ 611,392	$ 431,992
Percent of Revenue										52.70%	47.30%
After Reclassification | United Kingdom
Effect of Reclassification Adjustments [Line Items]
Revenues										$ 177,194	$ 166,418
Percent of Revenue										15.30%	18.20%
After Reclassification | Switzerland
Effect of Reclassification Adjustments [Line Items]
Revenues										$ 122,919	$ 111,686
Percent of Revenue										10.60%	12.20%
After Reclassification | Canada
Effect of Reclassification Adjustments [Line Items]
Revenues										$ 59,189	$ 58,319
Percent of Revenue										5.10%	6.40%
After Reclassification | Germany
Effect of Reclassification Adjustments [Line Items]
Revenues										$ 43,621	$ 36,403
Percent of Revenue										3.80%	4.00%
After Reclassification | Russia
Effect of Reclassification Adjustments [Line Items]
Revenues										$ 40,944	$ 36,585
Percent of Revenue										3.50%	4.00%
After Reclassification | Sweden
Effect of Reclassification Adjustments [Line Items]
Revenues										$ 23,838	$ 10,775
Percent of Revenue										2.10%	1.20%
After Reclassification | Hong Kong
Effect of Reclassification Adjustments [Line Items]
Revenues										$ 21,010	$ 23,285
Percent of Revenue										1.80%	2.50%
After Reclassification | Netherlands
Effect of Reclassification Adjustments [Line Items]
Revenues										$ 17,521	$ 10,373
Percent of Revenue										1.50%	1.10%
After Reclassification | Belgium
Effect of Reclassification Adjustments [Line Items]
Revenues										$ 8,627	$ 8,113
Percent of Revenue										0.70%	0.90%
After Reclassification | Ireland
Effect of Reclassification Adjustments [Line Items]
Revenues										$ 5,167	$ 5,536
Percent of Revenue										0.40%	0.60%
After Reclassification | China
Effect of Reclassification Adjustments [Line Items]
Revenues										$ 4,479	$ 817
Percent of Revenue										0.40%	0.10%
After Reclassification | Italy
Effect of Reclassification Adjustments [Line Items]
Revenues										$ 4,052	$ 2,342
Percent of Revenue										0.30%	0.30%
After Reclassification | United Arab Emirates
Effect of Reclassification Adjustments [Line Items]
Revenues										$ 3,615
Percent of Revenue										0.30%
After Reclassification | Other locations
Effect of Reclassification Adjustments [Line Items]
Revenues										$ 16,564	$ 11,484
Percent of Revenue										1.50%	1.20%