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Reportable Segments - Revenue and Total Assets by Geographic Area (Details)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Revenues and total assets by geographic area      
% of Revenue 100.00% 100.00% 100.00%
Non-United States      
Revenues and total assets by geographic area      
% of Revenue 2.90% 0.30% 0.60%
% of total assets 2.80% 1.00%