Concentration of Risk (Details) - customer	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Revenue | Customer concentration risk
Concentration of Risk
Number of customers	2	3	3	2
Revenue | Customer concentration risk | First customer
Concentration of Risk
Percentage of concentration risk	28.00%	17.80%	18.70%	22.90%
Revenue | Customer concentration risk | Second customer
Concentration of Risk
Percentage of concentration risk	13.20%	13.60%	12.40%	10.80%
Revenue | Customer concentration risk | Third customer
Concentration of Risk
Percentage of concentration risk		13.10%	11.80%
Consolidated accounts receivable | Customer concentration risk
Concentration of Risk
Number of customers			4
Consolidated accounts receivable | Credit concentration risk
Concentration of Risk
Number of customers					2
Consolidated accounts receivable | Credit concentration risk | First customer
Concentration of Risk
Percentage of concentration risk			21.10%		21.90%
Consolidated accounts receivable | Credit concentration risk | Second customer
Concentration of Risk
Percentage of concentration risk			17.50%		11.50%
Consolidated accounts receivable | Credit concentration risk | Third customer
Concentration of Risk
Percentage of concentration risk			11.20%
Consolidated accounts receivable | Credit concentration risk | Fourth customer
Concentration of Risk
Percentage of concentration risk			10.60%