Concentration of Risk (Details) - customer	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Revenue | Customer concentration risk
Concentration of Risk
Number of customers	4	1	4	2
Revenue | Customer concentration risk | First customer
Concentration of Risk
Percentage of concentration risk	15.70%	23.90%	14.80%	24.90%
Revenue | Customer concentration risk | Second customer
Concentration of Risk
Percentage of concentration risk	14.20%		14.40%	11.80%
Revenue | Customer concentration risk | Third customer
Concentration of Risk
Percentage of concentration risk	12.70%		11.40%
Revenue | Customer concentration risk | Fourth customer
Concentration of Risk
Percentage of concentration risk	12.50%		11.20%
Consolidated accounts receivable | Credit concentration risk
Concentration of Risk
Number of customers			3		2
Consolidated accounts receivable | Credit concentration risk | First customer
Concentration of Risk
Percentage of concentration risk			20.20%		21.90%
Consolidated accounts receivable | Credit concentration risk | Second customer
Concentration of Risk
Percentage of concentration risk			14.30%		11.50%
Consolidated accounts receivable | Credit concentration risk | Third customer
Concentration of Risk
Percentage of concentration risk			12.20%