Summary of Significant Accounting Policies - Concentration of Risk (Details) - item	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenue | Customer concentration risk
Concentration of risk
Number of customers	2	2	1
Revenue | Customer concentration risk | Ten largest customers
Concentration of risk
Number of customers	10	10	10
Percentage of concentration risk	76.80%	68.10%	69.10%
Revenue | Customer concentration risk | One customer
Concentration of risk
Percentage of concentration risk	29.30%	17.40%	15.50%
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	3	2
Consolidated accounts receivable | Credit concentration risk | One customer
Concentration of risk
Percentage of concentration risk	22.90%	21.70%
Consolidated accounts receivable | Credit concentration risk | Second customer
Concentration of risk
Percentage of concentration risk	12.70%	20.40%
Consolidated accounts receivable | Credit concentration risk | Third customer
Concentration of risk
Percentage of concentration risk	11.60%