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Geographic Information - Narrative (Details) - segment
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Concentration Risk [Line Items]        
Number of operating segments     1  
United States | Revenue | Geographic concentration risk        
Concentration Risk [Line Items]        
Concentration risk, percentage 66.00% 70.00% 67.00% 70.00%