BUSINESS INFORMATION (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($) item	Dec. 31, 2014 USD ($) item
Business information
Total revenues	$ 613,373	$ 551,863	$ 518,750
Total assets	$ 598,800	$ 539,461
Client | Revenue
Business information
Number of clients | item	0	0	0
Client | Revenue | Minimum
Business information
Concentration risk (as a percent)	10.00%	10.00%	10.00%
United States
Business information
Total revenues	$ 516,688	$ 443,998	$ 409,935
Total assets	506,748	463,984
Europe
Business information
Total revenues	54,332	88,063	89,797
Total assets	31,341	45,067
Rest Of World
Business information
Total revenues	42,353	19,802	$ 19,018
Total assets	$ 60,711	$ 30,410