SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Reclassification (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Item Effected [Line Items]
Revenues	$ 377,523	$ 298,293	$ 1,051,151	$ 846,607
Reimbursable expenses and other revenues
Item Effected [Line Items]
Revenues	4,134	3,159	12,226	8,731
United States
Item Effected [Line Items]
Revenues	202,364	155,241	571,513	439,244
United Kingdom
Item Effected [Line Items]
Revenues	48,471	44,179	141,177	133,391
Switzerland
Item Effected [Line Items]
Revenues	33,185	31,638	90,400	95,086
Canada
Item Effected [Line Items]
Revenues	15,711	14,589	41,854	45,762
Germany
Item Effected [Line Items]
Revenues	16,053	12,451	42,291	30,926
Russia
Item Effected [Line Items]
Revenues	14,079	9,861	40,614	29,331
Sweden
Item Effected [Line Items]
Revenues	8,491	6,506	24,134	16,667
Hong Kong
Item Effected [Line Items]
Revenues	5,185	4,991	14,453	16,218
Netherlands
Item Effected [Line Items]
Revenues	14,118	4,539	35,345	10,512
Belgium
Item Effected [Line Items]
Revenues	2,785	2,699	5,427	7,422
Ireland
Item Effected [Line Items]
Revenues	2,620	1,318	5,609	3,808
Italy
Item Effected [Line Items]
Revenues	1,654	1,125	4,451	2,799
China
Item Effected [Line Items]
Revenues	1,939	2,071	5,432	2,778
United Arab Emirates
Item Effected [Line Items]
Revenues	3,282	1,788	8,531	1,788
Other locations
Item Effected [Line Items]
Revenues	$ 7,586	5,297	$ 19,920	10,875
Previously reported
Item Effected [Line Items]
Revenues		$ 298,293		$ 846,607
Percent of Revenue		100.00%		100.00%
Previously reported | Reimbursable expenses and other revenues
Item Effected [Line Items]
Revenues		$ 3,159		$ 8,731
Percent of Revenue		1.10%		1.00%
Previously reported | United States
Item Effected [Line Items]
Revenues		$ 153,818		$ 435,258
Percent of Revenue		51.60%		51.40%
Previously reported | United Kingdom
Item Effected [Line Items]
Revenues		$ 43,535		$ 131,541
Percent of Revenue		14.60%		15.50%
Previously reported | Switzerland
Item Effected [Line Items]
Revenues		$ 31,550		$ 94,771
Percent of Revenue		10.60%		11.20%
Previously reported | Canada
Item Effected [Line Items]
Revenues		$ 14,526		$ 45,400
Percent of Revenue		4.90%		5.40%
Previously reported | Germany
Item Effected [Line Items]
Revenues		$ 12,325		$ 30,694
Percent of Revenue		4.10%		3.60%
Previously reported | Russia
Item Effected [Line Items]
Revenues		$ 9,833		$ 29,269
Percent of Revenue		3.30%		3.50%
Previously reported | Sweden
Item Effected [Line Items]
Revenues		$ 6,256		$ 16,204
Percent of Revenue		2.00%		1.90%
Previously reported | Hong Kong
Item Effected [Line Items]
Revenues		$ 4,839		$ 15,684
Percent of Revenue		1.60%		1.90%
Previously reported | Netherlands
Item Effected [Line Items]
Revenues		$ 4,302		$ 10,014
Percent of Revenue		1.40%		1.20%
Previously reported | Belgium
Item Effected [Line Items]
Revenues		$ 2,676		$ 7,329
Percent of Revenue		0.90%		0.90%
Previously reported | Ireland
Item Effected [Line Items]
Revenues		$ 1,315		$ 3,795
Percent of Revenue		0.40%		0.40%
Previously reported | Italy
Item Effected [Line Items]
Revenues		$ 1,103		$ 2,732
Percent of Revenue		0.40%		0.30%
Previously reported | China
Item Effected [Line Items]
Revenues		$ 2,050		$ 2,757
Percent of Revenue		0.70%		0.30%
Previously reported | United Arab Emirates
Item Effected [Line Items]
Revenues		$ 1,778		$ 1,778
Percent of Revenue		0.60%		0.20%
Previously reported | Other locations
Item Effected [Line Items]
Revenues		$ 5,228		$ 10,650
Percent of Revenue		1.80%		1.30%
After reclassification adjustment
Item Effected [Line Items]
Revenues		$ 298,293		$ 846,607
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		$ 155,241		$ 439,244
Percent of Revenue		52.00%		51.90%
After reclassification adjustment | United Kingdom
Item Effected [Line Items]
Revenues		$ 44,179		$ 133,391
Percent of Revenue		14.80%		15.80%
After reclassification adjustment | Switzerland
Item Effected [Line Items]
Revenues		$ 31,638		$ 95,086
Percent of Revenue		10.60%		11.20%
After reclassification adjustment | Canada
Item Effected [Line Items]
Revenues		$ 14,589		$ 45,762
Percent of Revenue		4.90%		5.40%
After reclassification adjustment | Germany
Item Effected [Line Items]
Revenues		$ 12,451		$ 30,926
Percent of Revenue		4.20%		3.70%
After reclassification adjustment | Russia
Item Effected [Line Items]
Revenues		$ 9,861		$ 29,331
Percent of Revenue		3.30%		3.50%
After reclassification adjustment | Sweden
Item Effected [Line Items]
Revenues		$ 6,506		$ 16,667
Percent of Revenue		2.20%		2.00%
After reclassification adjustment | Hong Kong
Item Effected [Line Items]
Revenues		$ 4,991		$ 16,218
Percent of Revenue		1.70%		1.90%
After reclassification adjustment | Netherlands
Item Effected [Line Items]
Revenues		$ 4,539		$ 10,512
Percent of Revenue		1.50%		1.20%
After reclassification adjustment | Belgium
Item Effected [Line Items]
Revenues		$ 2,699		$ 7,422
Percent of Revenue		0.90%		0.90%
After reclassification adjustment | Ireland
Item Effected [Line Items]
Revenues		$ 1,318		$ 3,808
Percent of Revenue		0.40%		0.40%
After reclassification adjustment | Italy
Item Effected [Line Items]
Revenues		$ 1,125		$ 2,799
Percent of Revenue		0.40%		0.30%
After reclassification adjustment | China
Item Effected [Line Items]
Revenues		$ 2,071		$ 2,778
Percent of Revenue		0.70%		0.30%
After reclassification adjustment | United Arab Emirates
Item Effected [Line Items]
Revenues		$ 1,788		$ 1,788
Percent of Revenue		0.60%		0.20%
After reclassification adjustment | Other locations
Item Effected [Line Items]
Revenues		$ 5,297		$ 10,875
Percent of Revenue		1.80%		1.30%