Concentration of Risk (Details) - customer	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Revenue | Customer concentration risk
Concentration of Risk
Number of customers	1	4	2	4
Revenue | Customer concentration risk | First customer
Concentration of Risk
Percentage of concentration risk	23.90%	27.30%	24.90%	22.30%
Revenue | Customer concentration risk | Second customer
Concentration of Risk
Percentage of concentration risk		21.60%	11.80%	19.20%
Revenue | Customer concentration risk | Third customer
Concentration of Risk
Percentage of concentration risk		12.80%		15.70%
Revenue | Customer concentration risk | Fourth customer
Concentration of Risk
Percentage of concentration risk		10.50%		13.30%
Consolidated accounts receivable | Credit concentration risk
Concentration of Risk
Number of customers			1		3
Consolidated accounts receivable | Credit concentration risk | First customer
Concentration of Risk
Percentage of concentration risk			10.00%		19.80%
Consolidated accounts receivable | Credit concentration risk | Second customer
Concentration of Risk
Percentage of concentration risk					11.80%
Consolidated accounts receivable | Credit concentration risk | Third customer
Concentration of Risk
Percentage of concentration risk					10.60%