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Customer Concentration (Details) - customer
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Customer concentrations | Sales          
Customer Concentration          
Number of customers accounted for more than 10% of sales individually 1 1 2 2  
Concentration risk 23.00% 22.00% 35.00% 30.00%  
Credit concentrations | Accounts receivable          
Customer Concentration          
Number of customers accounted for more than 10% of sales individually     1   1
Concentration risk     52.00%   43.00%