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Summary of significant accounting policies - Risk of Concentrations of Credit, Significant Customers and Significant Suppliers (Details) - Customers	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Total revenue | Major Customers
Concentration Risk [Line Items]
Customer concentration risk percentage	43.50%	59.60%	16.40%	48.40%
Total revenue | Customer A
Concentration Risk [Line Items]
Customer concentration risk percentage	32.10%	40.80%	16.40%	21.20%
Total revenue | Customer B
Concentration Risk [Line Items]
Customer concentration risk percentage				17.10%
Total revenue | Customer C
Concentration Risk [Line Items]
Customer concentration risk percentage				10.10%
Total revenue | Customer D
Concentration Risk [Line Items]
Customer concentration risk percentage	11.40%
Total revenue | Customer E
Concentration Risk [Line Items]
Customer concentration risk percentage		18.80%
Accounts receivable | Major Customers
Concentration Risk [Line Items]
Customer concentration risk percentage			53.00%		84.10%
Accounts receivable | Customer A
Concentration Risk [Line Items]
Customer concentration risk percentage			21.70%		41.90%
Accounts receivable | Customer D
Concentration Risk [Line Items]
Customer concentration risk percentage			20.20%
Accounts receivable | Customer E
Concentration Risk [Line Items]
Customer concentration risk percentage			11.10%
Accounts receivable | Customer F
Concentration Risk [Line Items]
Customer concentration risk percentage					10.10%
Accounts receivable | Customer G
Concentration Risk [Line Items]
Customer concentration risk percentage					13.40%
Accounts receivable | Customer H
Concentration Risk [Line Items]
Customer concentration risk percentage					18.70%

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