OTHER ASSETS AND ACCOUNTS PAYABLE, ACCRUED EXPENSES AND OTHER LIABILITIES - Accounts Payable, Accrued Expenses and Other Liabilities (Details) - USD ($) $ in Thousands, shares in Millions	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2015
OTHER ASSETS AND ACCOUNTS PAYABLE, ACCRUED EXPENSES AND OTHER LIABILITIES
Amounts Payable to Carry Pool	$ 1,100,943	$ 1,199,000
Unsettled Investment Purchases	891,649	594,152
Securities Sold Short	633,132	299,990
Accounts Payable and Accrued Expenses	119,491	112,007
Interest Payable	61,643	102,195
Foreign Exchange Contracts and Options	71,956	83,748
Derivative Liabilities	75,150	104,518
Contingent Consideration Obligation	40,600	46,600
Due to Broker	72,509	27,121
Deferred Rent and Income	26,894	21,706
Accrued Compensation and Benefits	17,799	17,765
Income Taxes Payable	6,362	8,770
Other Liabilities	81,224	97,778
Total	3,199,352	2,715,350
Securities sold, not yet purchased, cost basis	628,071	298,838
Foreign Exchange Contract
Derivative [Line Items]
Cost basis of derivative financial instruments, liabilities	0	$ 6,038
Prisma
Business Acquisition [Line Items]
Settlement of contingent consideration	123,600
Equity issued in connection with contingent payment obligation	$ 84,100
Units issued is connection with contingent payment obligation	3.7