REVENUE - REVENUE DISAGGREGATED BY REVENUE SOURCE (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Revenues	$ 451,167	$ 187,413	$ 871,606	$ 283,044
Securities lending	39,448	28,633	75,074	35,138
Margin interest	31,230	10,958	58,961	18,787
Interest on segregated cash and securities	931	1,436	2,041	10,632
Other interest revenue	1,368	526	2,197	2,516
Interest expenses related to credit facilities	(5,268)	(1,555)	(8,067)	(3,059)
Net interest revenues	67,709	39,998	130,206	64,014
Total net revenues	565,333	244,211	1,087,507	371,761
Options
Disaggregation of Revenue [Line Items]
Revenues	164,604	111,148	362,464	170,908
Cryptocurrencies
Disaggregation of Revenue [Line Items]
Revenues	233,103	5,320	320,690	9,558
Equities
Disaggregation of Revenue [Line Items]
Revenues	52,012	70,606	185,313	102,195
Other
Disaggregation of Revenue [Line Items]
Revenues	1,448	339	3,139	383
Other revenues
Disaggregation of Revenue [Line Items]
Revenues	$ 46,457	$ 16,800	$ 85,695	$ 24,703