Concentration of Risk (Details) - customer	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Revenue | Customer concentration risk
Concentration of Risk
Number of customers	3	2	3	3
Revenue | Customer concentration risk | First customer
Concentration of Risk
Percentage of concentration risk	16.80%	28.00%	18.80%	18.70%
Revenue | Customer concentration risk | Second customer
Concentration of Risk
Percentage of concentration risk	11.20%	13.20%	15.20%	12.40%
Revenue | Customer concentration risk | Third customer
Concentration of Risk
Percentage of concentration risk	10.30%		12.10%	11.80%
Consolidated accounts receivable | Credit concentration risk
Concentration of Risk
Number of customers			3		3
Consolidated accounts receivable | Credit concentration risk | First customer
Concentration of Risk
Percentage of concentration risk			19.50%		24.90%
Consolidated accounts receivable | Credit concentration risk | Second customer
Concentration of Risk
Percentage of concentration risk			12.90%		15.30%
Consolidated accounts receivable | Credit concentration risk | Third customer
Concentration of Risk
Percentage of concentration risk			10.20%		11.10%