Summary of Significant Accounting Policies - Concentration of Risk (Details) - item	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenue | Customer concentration risk
Concentration of risk
Number of customers	1	2	2
Revenue | Customer concentration risk | Ten largest customers
Concentration of risk
Number of customers	10	10	10
Percentage of concentration risk	70.20%	76.80%	68.10%
Revenue | Customer concentration risk | One customer
Concentration of risk
Percentage of concentration risk	17.00%	29.30%	17.40%
Revenue | Customer concentration risk | Second customer
Concentration of risk
Percentage of concentration risk		10.50%	12.30%
Consolidated accounts receivable | Credit concentration risk
Concentration of risk
Number of customers	4	3
Consolidated accounts receivable | Credit concentration risk | One customer
Concentration of risk
Percentage of concentration risk	22.00%	22.90%
Consolidated accounts receivable | Credit concentration risk | Second customer
Concentration of risk
Percentage of concentration risk	12.30%	12.70%
Consolidated accounts receivable | Credit concentration risk | Third customer
Concentration of risk
Percentage of concentration risk	12.00%	11.60%
Consolidated accounts receivable | Credit concentration risk | Fourth customer
Concentration of risk
Percentage of concentration risk	10.60%