Summary of Significant Accounting Policies - Concentration of Risk (Details) - customer	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue | Customer concentration risk
Concentration of risk
Number of customers	2	1	2
Revenue | Customer concentration risk | Ten largest customers
Concentration of risk
Number of customers	10	10	10
Percentage of concentration risk	73.30%	70.20%	76.80%
Revenue | Customer concentration risk | One customer
Concentration of risk
Percentage of concentration risk	24.90%	17.00%	29.30%
Revenue | Customer concentration risk | Second customer
Concentration of risk
Percentage of concentration risk	13.10%		10.50%
Consolidated accounts receivable | Credit concentration risk
Concentration of risk
Number of customers	3	4
Consolidated accounts receivable | Credit concentration risk | One customer
Concentration of risk
Percentage of concentration risk	19.80%	22.00%
Consolidated accounts receivable | Credit concentration risk | Second customer
Concentration of risk
Percentage of concentration risk	11.80%	12.30%
Consolidated accounts receivable | Credit concentration risk | Third customer
Concentration of risk
Percentage of concentration risk	10.60%	12.00%
Consolidated accounts receivable | Credit concentration risk | Fourth customer
Concentration of risk
Percentage of concentration risk		10.60%