DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - SCHEDULE OF CONCENTRATION OF CREDIT RISK (Details) - Customer Concentration Risk - Revenue Benchmark	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Citadel Securities, LLC
Concentration Risk [Line Items]
Concentration risk, percentage	23.00%	33.00%	21.00%	34.00%
Tai Mo Shan Limited
Concentration Risk [Line Items]
Concentration risk, percentage	8.00%	1.00%	17.00%	2.00%
Entities affiliated with Susquehanna International Group, LLP
Concentration Risk [Line Items]
Concentration risk, percentage	13.00%	18.00%	11.00%	20.00%
Entities affiliated with Wolverine Holdings, L.P.
Concentration Risk [Line Items]
Concentration risk, percentage	13.00%	10.00%	10.00%	10.00%
All others individually less than 10%
Concentration Risk [Line Items]
Concentration risk, percentage	16.00%	13.00%	19.00%	9.00%
Total as percentage of total revenue
Concentration Risk [Line Items]
Concentration risk, percentage	73.00%	75.00%	78.00%	75.00%