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Accounts receivable, unbilled revenue (contract assets) and unearned revenue or payments on account (contract liabilities) - Narrative (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]    
Amounts due to third parties for reimbursable expenses $ 323,600 $ 138,200
Change in unbilled receivables 194,437  
Change in unearned revenue (payments on account) 663,078  
Increase in advance payments netted against unbilled contracts receivable (468,641)  
Bad debt expense 900 $ 2,900
Revenue, remaining performance obligation, amount $ 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