SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Revenue recognition (Details) - USD ($)	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
IPO Distribution Services
Revenue from contracts with customers	$ 13,760,209	$ 3,142,763	$ 475,055

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

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+ Details