Carrying value as of the balance sheet date of liabilities incurred (and for which invoices have typically been received) and payable to vendors for goods and services received that are used in an entity's business. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer).

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Summary of Significant Accounting Policies - Concentration and risks (Details) - USD ($)	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Concentration and risks
Revenues	$ 339,065,455	$ 108,899,479
Accounts receivable, net	29,083,216		$ 27,254,634
Sales and marketing expenses	316,276,383	93,533,363
Accounts payable	64,805,107		$ 37,877,800
Customer concentration risk \| Revenue \| Company A
Concentration and risks
Revenues		$ 21,335,698
Concentration risk (as a percent)		20.00%
Customer concentration risk \| Revenue \| Company B
Concentration and risks
Revenues		$ 21,731,172
Concentration risk (as a percent)		20.00%
Customer concentration risk \| Revenue \| Company C
Concentration and risks
Revenues		$ 11,088,217
Concentration risk (as a percent)		10.00%
Customer concentration risk \| Revenue \| Company D
Concentration and risks
Revenues		$ 16,873,890
Concentration risk (as a percent)		16.00%
Customer concentration risk \| Revenue \| Company E
Concentration and risks
Revenues	$ 91,588,390
Concentration risk (as a percent)	27.00%
Customer concentration risk \| Accounts receivables \| Company B
Concentration and risks
Concentration risk (as a percent)		62.00%
Accounts receivable, net		$ 17,944,840
Customer concentration risk \| Accounts receivables \| Company C
Concentration and risks
Concentration risk (as a percent)		14.00%
Accounts receivable, net		$ 4,142,638
Customer concentration risk \| Accounts receivables \| Company D
Concentration and risks
Concentration risk (as a percent)		13.00%
Accounts receivable, net		$ 3,840,005
Customer concentration risk \| Accounts receivables \| Company E
Concentration and risks
Concentration risk (as a percent)	40.00%
Accounts receivable, net	$ 12,354,156
Customer concentration risk \| Accounts receivables \| Company F
Concentration and risks
Concentration risk (as a percent)	32.00%
Accounts receivable, net	$ 9,824,384
Vendor concentration risk \| Sales and marketing expenses \| Company G
Concentration and risks
Concentration risk (as a percent)		12.00%
Sales and marketing expenses		$ 11,080,305
Vendor concentration risk \| Sales and marketing expenses \| Company H
Concentration and risks
Concentration risk (as a percent)		12.00%
Sales and marketing expenses		$ 10,853,930
Vendor concentration risk \| Sales and marketing expenses \| Company I
Concentration and risks
Concentration risk (as a percent)	13.00%
Sales and marketing expenses	$ 41,804,056
Vendor concentration risk \| Accounts payable \| Company H
Concentration and risks
Concentration risk (as a percent)	13.00%
Accounts payable	$ 8,433,642
Vendor concentration risk \| Accounts payable \| Company J
Concentration and risks
Concentration risk (as a percent)	11.00%
Accounts payable	$ 7,378,433