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Contract Balances - Narrative (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2022
Dec. 31, 2021
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]    
Amounts due to third parties for reimbursable expenses $ 406,300 $ 323,600
Change in unbilled receivables 334,534  
Change in unearned revenue (payments on account) 183,488  
Increase in advance payments netted against unbilled contracts receivable 151,046  
Bad debt expense 17,800 900
Revenue, remaining performance obligation, amount $ 13,700,000 $ 13,300,000
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 (in percent)   48.00%
Revenue, remaining performance obligation, expected timing of satisfaction, period   12 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01    
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]    
Revenue, remaining performance obligation (in percent) 52.00%  
Revenue, remaining performance obligation, expected timing of satisfaction, period 12 months