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Revenue - Concentration of Major Clients (Details) - Revenue Benchmark - Customer Concentration Risk
9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Client 1    
Concentration Risk [Line Items]    
Concentration risk (as a percent) 16.00% 24.00%
Client 2    
Concentration Risk [Line Items]    
Concentration risk (as a percent) 10.00% 15.00%
Client 3    
Concentration Risk [Line Items]    
Concentration risk (as a percent) 10.00% 10.00%
Client 4    
Concentration Risk [Line Items]    
Concentration risk (as a percent) 12.00%