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Segments (Details) - segment
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Segment Reporting [Abstract]      
Number of operating segments 1    
Number of reportable segments 1    
Geographic Concentration Risk | Revenue | United States      
Concentration Risk [Line Items]      
Concentration risk percentage 92.10% 93.80% 94.30%