Concentration of Risk (Details) - customer	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Revenue | Customer concentration risk | One customer
Concentration of risk
Number of customers	4	3	4	2
Percentage of concentration risk	27.30%	14.80%	22.30%	16.40%
Revenue | Customer concentration risk | Second customer
Concentration of risk
Number of customers	4	3	4	2
Percentage of concentration risk	21.60%	11.70%	19.20%	14.10%
Revenue | Customer concentration risk | Third customer
Concentration of risk
Number of customers	4	3	4
Percentage of concentration risk	12.80%	10.40%	15.70%
Revenue | Customer concentration risk | Fourth customer
Concentration of risk
Number of customers	4		4
Percentage of concentration risk	10.50%		13.30%
Consolidated accounts receivable | Customer concentration risk | One customer
Concentration of risk
Number of customers			3		4
Consolidated accounts receivable | Customer concentration risk | Second customer
Concentration of risk
Number of customers			3		4
Consolidated accounts receivable | Customer concentration risk | Third customer
Concentration of risk
Number of customers			3		4
Consolidated accounts receivable | Customer concentration risk | Fourth customer
Concentration of risk
Number of customers					4
Consolidated accounts receivable | Credit concentration risk | One customer
Concentration of risk
Percentage of concentration risk			20.30%		22.00%
Consolidated accounts receivable | Credit concentration risk | Second customer
Concentration of risk
Percentage of concentration risk			18.20%		12.30%
Consolidated accounts receivable | Credit concentration risk | Third customer
Concentration of risk
Percentage of concentration risk			15.30%		12.00%
Consolidated accounts receivable | Credit concentration risk | Fourth customer
Concentration of risk
Percentage of concentration risk					10.60%