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Revenue - Concentration of Major Clients (Details) - Sales Revenue, Net - Customer Concentration Risk
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Concentration Risk [Line Items]      
Concentration risk (as a percent) 35.00% 41.00% 48.00%
Client 1      
Concentration Risk [Line Items]      
Concentration risk (as a percent) 18.00% 16.00% 24.00%
Client 2      
Concentration Risk [Line Items]      
Concentration risk (as a percent) 17.00% 15.00% 14.00%
Client 3      
Concentration Risk [Line Items]      
Concentration risk (as a percent)   10.00% 10.00%