REVENUE - REVENUE DISAGGREGATED BY REVENUE SOURCE (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Securities lending	$ 33,495	$ 27,891	$ 108,569	$ 63,029
Margin interest	34,238	21,288	93,199	40,075
Interest on segregated cash and securities	1,071	1,359	3,112	11,991
Other interest revenue	853	743	3,050	3,259
Interest expenses related to credit facilities	(6,252)	(875)	(14,319)	(3,934)
Net interest revenues	63,405	50,406	193,611	114,420
Total net revenues	364,923	269,530	1,452,430	641,291
Options
Disaggregation of Revenue [Line Items]
Revenues	163,750	126,769	526,214	297,677
Cryptocurrencies
Disaggregation of Revenue [Line Items]
Revenues	50,723	5,282	371,413	14,840
Equities
Disaggregation of Revenue [Line Items]
Revenues	50,459	68,818	235,772	171,013
Other
Disaggregation of Revenue [Line Items]
Revenues	1,865	938	5,004	1,321
Transaction-based revenues
Disaggregation of Revenue [Line Items]
Revenues	266,797	201,807	1,138,403	484,851
Other revenues
Disaggregation of Revenue [Line Items]
Revenues	$ 34,721	$ 17,317	$ 120,416	$ 42,020