Concentration of Risk (Details) - customer	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Revenue | Customer concentration risk
Concentration of Risk
Number of customers	3	2
Revenue | Customer concentration risk | First customer
Concentration of Risk
Percentage of concentration risk	16.40%	25.90%
Revenue | Customer concentration risk | Second customer
Concentration of Risk
Percentage of concentration risk	13.40%	15.40%
Revenue | Customer concentration risk | Third customer
Concentration of Risk
Percentage of concentration risk	10.20%
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	18.30%		21.90%
Consolidated accounts receivable | Credit concentration risk | Second customer
Concentration of Risk
Percentage of concentration risk	15.70%		11.50%
Consolidated accounts receivable | Credit concentration risk | Third customer
Concentration of Risk
Percentage of concentration risk	11.00%