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Summary of Significant Accounting Policies - Segment and Geographic Information (Details)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Concentration Risk [Line Items]      
Number of reportable segments 1    
Sales Revenue, Net | Geographic Concentration Risk | United States      
Concentration Risk [Line Items]      
Concentration risk, percentage 98.00% 98.00% 98.00%
Maximum | Noncurrent Assets | Geographic Concentration Risk | Outside of United States      
Concentration Risk [Line Items]      
Concentration risk, percentage 4.00% 1.00%