OTHER ASSETS AND ACCOUNTS PAYABLE, ACCRUED EXPENSES AND OTHER LIABILITIES - Accounts Payable, Accrued Expenses and Other Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
OTHER ASSETS AND ACCOUNTS PAYABLE, ACCRUED EXPENSES AND OTHER LIABILITIES
Amounts Payable to Carry Pool	$ 1,300,165	$ 1,220,559
Unsettled Investment Purchases	605,442	885,945
Securities Sold Short	513,442	692,007
Derivative Liabilities	40,738	69,381
Accrued Compensation and Benefits	404,797	35,953
Interest Payable	183,363	168,673
Foreign Exchange Contracts and Options	102,018	260,948
Accounts Payable and Accrued Expenses	126,261	152,916
Deferred Rent	13,997	17,441
Taxes Payable	70,040	35,933
Uncertain Tax Positions Reserve	59,798	58,369
Other Liabilities	265,257	56,125
Total	$ 3,685,318	$ 3,654,250