Concentration of Risk (Details) - customer	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Revenue | Customer concentration risk
Concentration of Risk
Number of customers	3		2	4
Revenue | Customer concentration risk | First customer
Concentration of Risk
Percentage of concentration risk	17.80%	25.90%	22.90%	23.60%
Revenue | Customer concentration risk | Second customer
Concentration of Risk
Percentage of concentration risk	13.60%	15.90%	10.80%	18.00%
Revenue | Customer concentration risk | Third customer
Concentration of Risk
Percentage of concentration risk	13.10%			13.40%
Revenue | Customer concentration risk | Fourth customer
Concentration of Risk
Percentage of concentration risk				10.50%
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			16.00%		19.80%
Consolidated accounts receivable | Credit concentration risk | Second customer
Concentration of Risk
Percentage of concentration risk			13.80%		11.80%
Consolidated accounts receivable | Credit concentration risk | Third customer
Concentration of Risk
Percentage of concentration risk			12.50%		10.60%