Summary of Significant Accounting Policies - Concentration (Details) - customer	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Sales | Customer
Risk and Uncertainties
Number of customers exceeding 10% of benchmark	0	0	0
Accounts Receivable | Credit
Risk and Uncertainties
Number of customers exceeding 10% of benchmark	0	0