Summary of Significant Accounting Policies - Concentration of Risk (Details) - customer	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue | Customer concentration risk
Concentration of Risk
Number of customers	2	3	2
Revenue | Customer concentration risk | Ten largest customers
Concentration of Risk
Number of customers	10	10	10
Percentage of concentration risk	74.00%	74.10%	76.90%
Revenue | Customer concentration risk | First customer
Concentration of Risk
Percentage of concentration risk	17.90%	18.20%	20.10%
Revenue | Customer concentration risk | Second customer
Concentration of Risk
Percentage of concentration risk	16.40%	14.20%	12.10%
Revenue | Customer concentration risk | Third customer
Concentration of Risk
Percentage of concentration risk		12.00%
Consolidated accounts receivable | Credit concentration risk
Concentration of Risk
Number of customers	2	3
Consolidated accounts receivable | Credit concentration risk | First customer
Concentration of Risk
Percentage of concentration risk	23.80%	24.90%
Consolidated accounts receivable | Credit concentration risk | Second customer
Concentration of Risk
Percentage of concentration risk	11.90%	15.30%
Consolidated accounts receivable | Credit concentration risk | Third customer
Concentration of Risk
Percentage of concentration risk		11.10%