Concentration of Risk (Details) - customer	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Revenue | Customer concentration risk
Concentration of risk
Number of customers	2	4
Revenue | Customer concentration risk | First customer
Concentration of risk
Percentage of concentration risk	25.90%	19.20%
Revenue | Customer concentration risk | Second customer
Concentration of risk
Percentage of concentration risk	15.40%	16.60%
Revenue | Customer concentration risk | Third customer
Concentration of risk
Percentage of concentration risk		16.40%
Revenue | Customer concentration risk | Fourth customer
Concentration of risk
Percentage of concentration risk		16.30%
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	18.10%		19.80%
Consolidated accounts receivable | Credit concentration risk | Second customer
Concentration of risk
Percentage of concentration risk	17.30%		11.80%
Consolidated accounts receivable | Credit concentration risk | Third customer
Concentration of risk
Percentage of concentration risk	11.10%		10.60%