SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Reclassification (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Item Effected [Line Items]
Revenues	$ 348,977	$ 283,832	$ 673,628	$ 548,314
Reimbursable expenses and other revenues
Item Effected [Line Items]
Revenues	4,167	3,126	8,092	5,572
United States
Item Effected [Line Items]
Revenues	192,319	147,364	369,149	284,003
United Kingdom
Item Effected [Line Items]
Revenues	46,112	45,652	92,706	89,212
Switzerland
Item Effected [Line Items]
Revenues	28,119	32,596	57,215	63,448
Canada
Item Effected [Line Items]
Revenues	13,649	15,327	26,143	31,173
Russia
Item Effected [Line Items]
Revenues	13,326	10,505	26,535	19,470
Germany
Item Effected [Line Items]
Revenues	14,144	9,736	26,238	18,475
Hong Kong
Item Effected [Line Items]
Revenues	4,491	5,283	9,268	11,227
Sweden
Item Effected [Line Items]
Revenues	8,177	6,033	15,643	10,161
Netherlands
Item Effected [Line Items]
Revenues	12,556	3,326	21,227	5,973
Belgium
Item Effected [Line Items]
Revenues	1,398	2,571	2,642	4,723
Ireland
Item Effected [Line Items]
Revenues	1,600	1,304	2,989	2,490
Italy
Item Effected [Line Items]
Revenues	1,517	1,006	2,797	1,674
China
Item Effected [Line Items]
Revenues	1,909	413	3,493	707
Other locations
Item Effected [Line Items]
Revenues	$ 6,459	2,716	$ 12,334	5,578
Previously reported
Item Effected [Line Items]
Revenues		$ 283,832		$ 548,314
Percent of Revenue		100.00%		100.00%
Previously reported | Reimbursable expenses and other revenues
Item Effected [Line Items]
Revenues		$ 3,126		$ 5,572
Percent of Revenue		1.10%		1.00%
Previously reported | United States
Item Effected [Line Items]
Revenues		$ 145,882		$ 281,440
Percent of Revenue		51.40%		51.30%
Previously reported | United Kingdom
Item Effected [Line Items]
Revenues		$ 45,016		$ 88,006
Percent of Revenue		15.90%		16.10%
Previously reported | Switzerland
Item Effected [Line Items]
Revenues		$ 32,456		$ 63,221
Percent of Revenue		11.40%		11.50%
Previously reported | Canada
Item Effected [Line Items]
Revenues		$ 15,173		$ 30,874
Percent of Revenue		5.30%		5.60%
Previously reported | Russia
Item Effected [Line Items]
Revenues		$ 10,484		$ 19,436
Percent of Revenue		3.70%		3.50%
Previously reported | Germany
Item Effected [Line Items]
Revenues		$ 9,663		$ 18,369
Percent of Revenue		3.40%		3.40%
Previously reported | Hong Kong
Item Effected [Line Items]
Revenues		$ 5,091		$ 10,845
Percent of Revenue		1.80%		2.00%
Previously reported | Sweden
Item Effected [Line Items]
Revenues		$ 5,888		$ 9,948
Percent of Revenue		2.10%		1.80%
Previously reported | Netherlands
Item Effected [Line Items]
Revenues		$ 3,165		$ 5,712
Percent of Revenue		1.10%		1.00%
Previously reported | Belgium
Item Effected [Line Items]
Revenues		$ 2,529		$ 4,653
Percent of Revenue		0.90%		0.90%
Previously reported | Ireland
Item Effected [Line Items]
Revenues		$ 1,296		$ 2,480
Percent of Revenue		0.50%		0.50%
Previously reported | Italy
Item Effected [Line Items]
Revenues		$ 993		$ 1,629
Percent of Revenue		0.40%		0.30%
Previously reported | China
Item Effected [Line Items]
Revenues		$ 413		$ 707
Percent of Revenue		0.10%		0.10%
Previously reported | Other locations
Item Effected [Line Items]
Revenues		$ 2,657		$ 5,422
Percent of Revenue		0.90%		1.00%
After reclassification adjustment
Item Effected [Line Items]
Revenues		$ 283,832		$ 548,314
Percent of Revenue		100.00%		100.00%
After reclassification adjustment | Reimbursable expenses and other revenues
Item Effected [Line Items]
Revenues		$ 0		$ 0
Percent of Revenue		0.00%		0.00%
After reclassification adjustment | United States
Item Effected [Line Items]
Revenues		$ 147,364		$ 284,003
Percent of Revenue		51.90%		51.80%
After reclassification adjustment | United Kingdom
Item Effected [Line Items]
Revenues		$ 45,652		$ 89,212
Percent of Revenue		16.10%		16.30%
After reclassification adjustment | Switzerland
Item Effected [Line Items]
Revenues		$ 32,596		$ 63,448
Percent of Revenue		11.50%		11.60%
After reclassification adjustment | Canada
Item Effected [Line Items]
Revenues		$ 15,327		$ 31,173
Percent of Revenue		5.40%		5.70%
After reclassification adjustment | Russia
Item Effected [Line Items]
Revenues		$ 10,505		$ 19,470
Percent of Revenue		3.70%		3.50%
After reclassification adjustment | Germany
Item Effected [Line Items]
Revenues		$ 9,736		$ 18,475
Percent of Revenue		3.40%		3.40%
After reclassification adjustment | Hong Kong
Item Effected [Line Items]
Revenues		$ 5,283		$ 11,227
Percent of Revenue		1.90%		2.00%
After reclassification adjustment | Sweden
Item Effected [Line Items]
Revenues		$ 6,033		$ 10,161
Percent of Revenue		2.10%		1.90%
After reclassification adjustment | Netherlands
Item Effected [Line Items]
Revenues		$ 3,326		$ 5,973
Percent of Revenue		1.20%		1.00%
After reclassification adjustment | Belgium
Item Effected [Line Items]
Revenues		$ 2,571		$ 4,723
Percent of Revenue		0.90%		0.90%
After reclassification adjustment | Ireland
Item Effected [Line Items]
Revenues		$ 1,304		$ 2,490
Percent of Revenue		0.50%		0.50%
After reclassification adjustment | Italy
Item Effected [Line Items]
Revenues		$ 1,006		$ 1,674
Percent of Revenue		0.40%		0.30%
After reclassification adjustment | China
Item Effected [Line Items]
Revenues		$ 413		$ 707
Percent of Revenue		0.10%		0.10%
After reclassification adjustment | Other locations
Item Effected [Line Items]
Revenues		$ 2,716		$ 5,578
Percent of Revenue		0.90%		1.00%