SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Concentration of supplier (Details)	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Total net revenues were executed and cleared by one supplier \| Concentration of supplier
Concentration risk
Concentration risk (in percent)	74.80%	78.40%	96.80%

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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