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Revenue - Contract Assets and Liabilities (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Revenue from Contract with Customer [Abstract]    
Unbilled service fees $ 49,110 $ 65,004
Unbilled outside vendor costs, billable to clients 10,552 30,133
Capitalized Contract Cost [Line Items]    
Advance billings 112,755 216,931
Decrease in deferred revenues 104,176  
Revenue recognized 210,078  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]    
Revenue, Remaining Performance Obligation, Amount $ 6,105  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01    
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]    
Revenue, Remaining Performance Obligation, Percentage 92.00%  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period 1 year  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01    
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]    
Revenue, Remaining Performance Obligation, Percentage 8.00%  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period 1 year  
Advance Billings [Member]    
Capitalized Contract Cost [Line Items]    
Advance billings $ 152,956 $ 171,742
Decrease in deferred revenues (18,786)  
Revenue recognized $ 152,361