Concentration of Risk (Details) - customer	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Revenue | Customer concentration risk | One customer
Concentration of risk
Number of customers	2	3	4	2
Percentage of concentration risk	25.90%	16.00%	23.60%	12.30%
Revenue | Customer concentration risk | Second customer
Concentration of risk
Number of customers	2	3	4	2
Percentage of concentration risk	15.90%	12.00%	18.00%	10.10%
Revenue | Customer concentration risk | Third customer
Concentration of risk
Number of customers		3	4
Percentage of concentration risk		11.70%	13.40%
Revenue | Customer concentration risk | Fourth customer
Concentration of risk
Number of customers			4
Percentage of concentration risk			10.50%
Consolidated accounts receivable | Credit concentration risk | One customer
Concentration of risk
Number of customers			2		4
Percentage of concentration risk			22.90%		22.00%
Consolidated accounts receivable | Credit concentration risk | Second customer
Concentration of risk
Number of customers			2		4
Percentage of concentration risk			14.70%		12.30%
Consolidated accounts receivable | Credit concentration risk | Third customer
Concentration of risk
Number of customers					4
Percentage of concentration risk					12.00%
Consolidated accounts receivable | Credit concentration risk | Fourth customer
Concentration of risk
Number of customers					4
Percentage of concentration risk					10.60%