OTHER ASSETS AND ACCOUNTS PAYABLE, ACCRUED EXPENSES AND OTHER LIABILITIES - Accounts Payable, Accrued Expenses and Other Liabilities (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
OTHER ASSETS AND ACCOUNTS PAYABLE, ACCRUED EXPENSES AND OTHER LIABILITIES
Amounts Payable to Carry Pool	$ 1,393,381	$ 1,448,879
Unsettled Investment Purchases	1,352,482	481,337
Securities Sold Short	434,598	251,223
Derivative Liabilities	115,221	34,174
Accrued Compensation and Benefits	558,179	131,719
Interest Payable	177,434	234,165
Foreign Exchange Contracts and Options	263,927	39,364
Accounts Payable and Accrued Expenses	156,963	118,454
Taxes Payable	36,484	32,682
Uncertain Tax Positions	68,665	65,716
Unfunded Revolver Commitments	56,085	75,842
Operating Lease Liabilities	161,165	125,086
Other Liabilities	86,534	58,922
Total	$ 4,861,118	$ 3,097,563
Lessee, Lease, Description [Line Items]
Operating lease, maximum extension term (in years)	3 years
Operating lease, weighted average remaining lease term (in years)	8 years 9 months 10 days	4 years 5 months 15 days
Operating lease, weighted average discount rate (as a percentage)	1.33%	2.53%
Minimum
Lessee, Lease, Description [Line Items]
Operating lease, remaining lease term (in years)	1 year
Maximum
Lessee, Lease, Description [Line Items]
Operating lease, remaining lease term (in years)	15 years