SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Reclassification (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Item Effected [Line Items]
Revenues	$ 324,651	$ 264,482
Reimbursable expenses and other revenues
Item Effected [Line Items]
Revenues	3,925	2,446
United States
Item Effected [Line Items]
Revenues	176,830	136,639
United Kingdom
Item Effected [Line Items]
Revenues	46,594	43,560
Switzerland
Item Effected [Line Items]
Revenues	29,096	30,852
Canada
Item Effected [Line Items]
Revenues	12,494	15,846
Russia
Item Effected [Line Items]
Revenues	13,209	8,965
Germany
Item Effected [Line Items]
Revenues	12,094	8,739
Hong Kong
Item Effected [Line Items]
Revenues	4,777	5,944
Sweden
Item Effected [Line Items]
Revenues	7,466	4,128
Netherlands
Item Effected [Line Items]
Revenues	8,671	2,647
Ireland
Item Effected [Line Items]
Revenues	1,389	1,186
Italy
Item Effected [Line Items]
Revenues	1,280	668
Denmark
Item Effected [Line Items]
Revenues	1,253	410
China
Item Effected [Line Items]
Revenues	1,584	294
Other locations
Item Effected [Line Items]
Revenues	$ 5,866	4,604
Previously reported
Item Effected [Line Items]
Revenues		$ 264,482
Percent of Revenue		100.00%
Previously reported | Reimbursable expenses and other revenues
Item Effected [Line Items]
Revenues		$ 2,446
Percent of Revenue		0.90%
Previously reported | United States
Item Effected [Line Items]
Revenues		$ 135,558
Percent of Revenue		51.30%
Previously reported | United Kingdom
Item Effected [Line Items]
Revenues		$ 42,990
Percent of Revenue		16.30%
Previously reported | Switzerland
Item Effected [Line Items]
Revenues		$ 30,765
Percent of Revenue		11.60%
Previously reported | Canada
Item Effected [Line Items]
Revenues		$ 15,701
Percent of Revenue		5.90%
Previously reported | Russia
Item Effected [Line Items]
Revenues		$ 8,952
Percent of Revenue		3.40%
Previously reported | Germany
Item Effected [Line Items]
Revenues		$ 8,706
Percent of Revenue		3.30%
Previously reported | Hong Kong
Item Effected [Line Items]
Revenues		$ 5,754
Percent of Revenue		2.20%
Previously reported | Sweden
Item Effected [Line Items]
Revenues		$ 4,060
Percent of Revenue		1.50%
Previously reported | Netherlands
Item Effected [Line Items]
Revenues		$ 2,547
Percent of Revenue		1.00%
Previously reported | Ireland
Item Effected [Line Items]
Revenues		$ 1,184
Percent of Revenue		0.40%
Previously reported | Italy
Item Effected [Line Items]
Revenues		$ 636
Percent of Revenue		0.20%
Previously reported | Denmark
Item Effected [Line Items]
Revenues		$ 405
Percent of Revenue		0.20%
Previously reported | China
Item Effected [Line Items]
Revenues		$ 294
Percent of Revenue		0.10%
Previously reported | Other locations
Item Effected [Line Items]
Revenues		$ 4,484
Percent of Revenue		1.70%
After reclassification adjustment
Item Effected [Line Items]
Revenues		$ 264,482
Percent of Revenue		100.00%
After reclassification adjustment | Reimbursable expenses and other revenues
Item Effected [Line Items]
Revenues		$ 0
Percent of Revenue		0.00%
After reclassification adjustment | United States
Item Effected [Line Items]
Revenues		$ 136,639
Percent of Revenue		51.70%
After reclassification adjustment | United Kingdom
Item Effected [Line Items]
Revenues		$ 43,560
Percent of Revenue		16.50%
After reclassification adjustment | Switzerland
Item Effected [Line Items]
Revenues		$ 30,852
Percent of Revenue		11.70%
After reclassification adjustment | Canada
Item Effected [Line Items]
Revenues		$ 15,846
Percent of Revenue		6.00%
After reclassification adjustment | Russia
Item Effected [Line Items]
Revenues		$ 8,965
Percent of Revenue		3.40%
After reclassification adjustment | Germany
Item Effected [Line Items]
Revenues		$ 8,739
Percent of Revenue		3.30%
After reclassification adjustment | Hong Kong
Item Effected [Line Items]
Revenues		$ 5,944
Percent of Revenue		2.20%
After reclassification adjustment | Sweden
Item Effected [Line Items]
Revenues		$ 4,128
Percent of Revenue		1.50%
After reclassification adjustment | Netherlands
Item Effected [Line Items]
Revenues		$ 2,647
Percent of Revenue		1.00%
After reclassification adjustment | Ireland
Item Effected [Line Items]
Revenues		$ 1,186
Percent of Revenue		0.40%
After reclassification adjustment | Italy
Item Effected [Line Items]
Revenues		$ 668
Percent of Revenue		0.30%
After reclassification adjustment | Denmark
Item Effected [Line Items]
Revenues		$ 410
Percent of Revenue		0.20%
After reclassification adjustment | China
Item Effected [Line Items]
Revenues		$ 294
Percent of Revenue		0.10%
After reclassification adjustment | Other locations
Item Effected [Line Items]
Revenues		$ 4,604
Percent of Revenue		1.70%